EGF receptor

Effects of Mutation or Deletion

Table of contents

Egfr and oogenesis

Gurken signals from the oocyte to the adjacent follicle cells twice during Drosophila oogenesis; first to induce posterior fate, thereby polarizing the anterior-posterior axis of the future embryo and then to induce dorsal fate and polarize the dorsal-ventral axis. Gurken is here shown to induce two different follicle cell fates because the follicle cells at the termini of the egg chamber differ in their competence to respond to Gurken from the main-body follicle cells in between. Anterior follicle cells are known to become subdivided into three distinct follicle cell types along the anterior-posterior axis: border cells, stretched follicle cells and centripetal follicle cells. The border cells are a group of 6-10 cells that delaminate from the follicular epithelium at the anterior tip of the egg chamber and migrate between the nurse cells to the anterior of the oocyte. At the same time, the adjacent stretched follicle cells spread to cover the nurse cells as the rest of the follicular epithelium moves posteriorly to envelop the oocyte. The centripetal follicle cells just posterior to the stretched follicle cells come to lie over the anterior of the oocyte after these movements are complete, and these cells then migrate between the oocyte and the nurse cells toward the center of the egg chamber during stage 10b (Gonzalez-Reyes, 1998).

It is argued that the terminal follicle cell populations (consisting of both anterior and posterior follicle cell populations) are equivalent prior to gurken signaling. To explain how Gurken can induce two different follicle cell fates, it has been proposed that the follicle cell layer is divided into two cell types during early oogenesis: the terminal follicle cells at each end of the egg chamber, which become posterior if they receive the Gurken signal and anterior if they do not, and the main-body follicle cells, which are induced to become dorsal rather than ventral. The Egfr, as receptor of the posterior Gurken signal, is required cell autonomously to repress anterior fate in all posterior follicle cells. Although the expression of several markers at the termini of developing egg chambers suggests the existence of populations of terminal follicle cells, it is not clear how many cells respond to Gurken directly by adopting a posterior rather than an anterior fate. To define this population, a mapping was performed to determine which cells revert to the default anterior fate when they cannot respond to Gurken because they lack its putative receptor. Small marked clones of cells were generated that are homozygous for top CO, a null allele of the Egfr, and their fate was followed by staining for the beta-gal activity of the L53b enhancer trap line, which labels all three subpopulations of anterior follicle cells from stage 9 onwards. When the clones are generated (at approximately stage 2 of oogenesis) and scored at stage 10, mutant cells that lie near the posterior of the oocyte are seen to always express L53b, whereas clones over the middle of the oocyte do not. Thus, removal of the Egfr causes a cell-autonomous transformation from posterior to anterior fate, indicating that Gurken signals directly to induce posterior fate in the whole terminal follicle cell population. With one exception, all Egfr- cells that fall within 10-11 cell diameters of the posterior end of the egg chamber express L53b, whereas mutant cells that fall anterior to this boundary do not. This analysis indicates that about 200 terminal follicle cells receive the Gurken signal directly, ruling out a model in which only the polar follicle cells (the most posterior cell population) are competent to respond to Gurken by becoming posterior. The cells that become anterior if they cannot respond to Gurken constitute the entire population of follicle cells that contact the oocyte during previtellogenic stages. Thus, all of the cells that receive the ‘posteriorizing’ Gurken signal are competent to respond to it (Gonzalez-Reyes, 1998).

In mutants such as gurken in which the induction of posterior follicle cell fate is blocked, the terminal follicle cells at the posterior develop like their anterior counterparts by forming border, stretched and centripetal follicle cells. This raises the question of whether the anterior follicle cells are subdivided into three cell types after the decision between anterior and posterior is taken, or whether there is a symmetric prepattern in the terminal follicle cells at both ends of the egg chamber. The ability to generate small clones of anterior cells at the posterior by removing the Egfr makes it possible to distinguish between these possibilities. If the latter model is correct, isolated patches of anterior cells should still respond to the symmetric prepattern correctly and form the appropriate type of anterior cell, even though they are surrounded by posterior cells, whereas the former model predicts that these cells should be unable to interpret their position. To follow the fate of small patches of anterior cells at the posterior of the egg chamber, small Egfr- clones were generated, but in this case, clone generation took place in the presence of enhancer trap lines that are expressed specifically in each of the three anterior follicle cell types. Egfr- cells that fall within a region 8-11 cell diameters from the posterior pole show staining for a centripetal cell marker, whereas clones that fall either proximal or distal to this 3-cell-wide belt do not activate this marker. Thus, anterior cells at the posterior express the anterior BB127 centripetal cell marker autonomously in a region that is the exact posterior counterpart of the anterior centripetal follicle cell domain. Furthermore, clones of as few as 4 cells express BB127 if they fall within this region, indicating that anterior cells can correctly interpret their position with respect to the posterior pole, although all of the surrounding cells are posterior. The same conclusion applies to a border cell and a stretched cell marker. The results demonstrate that small posterior clones of anterior cells can interpret their position with respect to the posterior pole by adopting the appropriate anterior follicle cell fate: the most terminal Egfr- cells behave like border cells, the subterminal Egfr- cells behave like stretched follicle cells, and the least terminal like centripetal cells. Thus, the positional information that specifies the positions of these distinct cell types at the anterior pole is also present at the posterior, strongly suggesting that there is a symmetric prepattern within the terminal follicle cell population that is independent of the decision between anterior and posterior fate (Gonzalez-Reyes, 1998).

These results suggest a three-step model for the anterior-posterior patterning of the follicular epithelium that subdivides this axis into at least five distinct cell types. Altogether, these observations support a stepwise model for the patterning of the follicle cell layer along the AP axis. In the first step, the follicle cell epithelium is divided into terminal and main-body follicle cell populations. There is no lineage restriction boundary between the posterior terminal follicle cells and the main-body follicle cells at a stage in development that is four cell divisions before stage 6, indicating that the distinction between these two cell types arises after stage 1. Because the terminal cells have to be specified before Gurken signaling occurs, this restricts the time at which this population is determined to between stages 2 and 5. Although the data do not suggest a mechanism for how these cells are specified, their position suggests a simple model in which they are induced by a ‘terminalizing’ signal that spreads from the two poles of the egg chamber. The most likely sources for such a signal are the two polar follicle cells at each end of the egg chamber, since these cells lie in the center of the terminal domain and adopt a terminal fate themselves. The next step in the patterning of the follicular epithelium is the formation of a symmetric prepattern within each terminal follicle cell population. How this prepattern is established is unknown, but the geometry of the egg chamber again suggests that it might involve signals that emanate from the poles. Indeed, it is possible that the terminal follicle cells are specified and patterned by the same process, since both events require Notch activity. For example, the ‘terminalizing’ signal could induce distinct terminal fates at different distances from the pole. The third step in the patterning of the follicle cell layer occurs when the oocyte induces one population of terminal follicle cells to adopt a posterior fate, thereby breaking the symmetry of the follicle cell layer. As a consequence, the symmetric prepattern in the terminal follicle cells is interpreted differently in the anterior and posterior populations. The anterior cells become subdivided into border, stretched and centripetal follicle cells, while the posterior cells may undergo a similar subdivision into posterior cell types. In this way, the sequential patterning of the terminal follicle cells gives rise to at least five different cell types along the anterior-posterior axis (Gonzalez-Reyes, 1998).

During Drosophila oogenesis the body axes are determined by signaling between the oocyte and the somatic follicle cells that surround the egg chamber. A key event in the establishment of oocyte anterior-posterior polarity is the differential patterning of the follicle cell epithelium along the anterior-posterior axis. Both the Notch and epithelial growth factor (EGF) receptor pathways are required for this patterning. To understand how these pathways act in the process, an examination was made using markers for anterior and posterior follicle cells accompanying constitutive activation of the EGF receptor, loss of Notch function, and ectopic expression of Delta. A constitutively active EGF receptor can induce posterior fate in anterior but not in lateral follicle cells, showing that the EGF receptor pathway can act only on predetermined terminal cells. Furthermore, Notch function is required at both termini for appropriate expression of anterior and posterior markers, while loss of both the EGF receptor and Notch pathways mimic the Notch loss-of-function phenotype. Ectopic expression of the Notch ligand, Delta, disturbs EGF receptor dependent posterior follicle cell differentiation and anterior-posterior polarity of the oocyte. These data are consistent with a model in which the Notch pathway is required for early follicle cell differentiation at both termini, but is then repressed at the posterior for proper determination of the posterior follicle cells by the EGF receptor pathway (Larkin, 1999).

To further investigate the interplay between Notch and Delta in follicle cell differentiation the effect of overexpression of Delta in the germarium was studied. The Drosophila ovary consists of 15-20 ovarioles: strings of egg chambers aligned in developmental order. At the anterior end of each ovariole lies the germarium, where the germ line stem cells divide to form 16 cell cysts. These cysts are enveloped by a somatic follicle cell layer and released from the germarium as a subset of follicle cells intercalates to form an interfollicular stalk. Expression of constitutively active Notch generates long stalks in the germarium by virtue of holding the stalk cells and polar cells in a precursor stage. Loss of Notch or Delta activity results in the opposite phenotype: lack of stalks. Overexpression of Delta in the germarium leads to the formation of long stalklike structures. These long stalks do not contain differentiated stalk or polar cells. Instead the markers Fasciclin III (FasIII) and Big Brain (Bib) are expressed as in the stalk cell precursors. These data suggest that overexpression of Delta produces long stalks due to a prolonged precursor stage for stalk and polar cells, a phenotype observed previously due to expression of constitutively active Notch; thus the phenotype produced by overexpression of Delta mimics that of the constitutively active Notch receptor in this developmental process (Larkin, 1999).

The data presented here show that the function of the EGF receptor pathway in posterior follicle cells requires functional Notch, but that the Notch pathway can act in these cells without an active EGF receptor pathway. A target for the EGF receptor pathway, pointed P1 is not activated in temperature sensitive Notch egg chambers, but the Notch-dependent termini are established in grk mutants. In addition, in Notch and grk double mutant experiments, the Notch loss-of-function phenotype is observed. However, if a ligand for Notch, Delta is overproduced at stage 6, posterior follicle cell development is compromised. The simplest model to explain these data is one in which the Notch pathway acts in both termini for differentiation of the terminal follicle cells and is subsequently repressed at the posterior for the EGF receptor-dependent posterior follicle cell differentiation. Therefore, proper function of the EGF receptor pathway in the posterior follicle cells requires the cessation of Delta expression in these follicle cells, suggesting that the Notch pathway can modulate cellular responses to the EGF receptor pathway (Larkin, 1999).

In Drosophila, the dorsoventral asymmetry of the egg chamber depends on a dorsalizing signal that emanates from the oocyte. This signal is supplied by the TGF alpha-like Gurken protein whose RNA is localized to the dorsal-anterior corner of the oocyte. Gurken protein expressed in the follicular epithelium surrounding the oocyte is the potential ligand of the Drosophila EGF receptor. Changes in the dorsalizing germ-line signal affect the embryonic dorsoventral pattern. A reduction in strength of the germ-line signal as produced by mutations in gurken or Egfr does not change the slope of the embryonic dorsoventral morphogen gradient, but causes a splitting of the gradient ventrally. This leads to embryos with two partial dorsoventral axes (Roth, 1994).

A set of dorsal follicle cells is patterned by the oocyte in a cell-cell signaling event occurring at stages 8 and 9 when the germinal vesicle (nucleus) migrates to the dorsal anterior of the oocyte. The anterodorsally positioned oocyte nucleus produces Gurken mRNA, a proposed ligand for the Epidermal growth cell receptor gene present on the overlying follicle cells. Activating Egfr transmits a signal through a Raf-dependent signaling pathway to generate anterior dorsal follicle cell fates, resulting in the respective specializations of the eggshell, including the dorsal appendages. A ventral follicle cell subpopulation that does not experience induction by Gurken produces molecular cues for a different inductive event, directing embryonic dorsal-ventral embryonic axis formation (Dobens, 1997 and references).

A Drosophila sequence homologous to the mammalian growth factor-stimulated TSC-22 gene was isolated in an enhancer trap screen for genes expressed in anterodorsal follicle cells during oogenesis. In situ hybridization reveals that bunched transcripts localize to the anterior dorsal follicle cells at stages 10-12 of oogenesis. Additional staining is evident in the border cells at the nurse cell/oocyte border and in a group of posterior polar follicle cells. The centripetally migrating follicle cells, just anterior to the stained columnar cells of the anterodorsal patch do not stain. Changes in bun enhancer trap expression in genetic backgrounds that disrupt the grk/Egfr signaling pathway suggest that bun is regulated by growth factor patterning of dorsal anterior follicle cell fates. In fs(1)K10 mutant egg chambers, dorsal follicle cell fates expand at the expense of ventral follicle cell fates, presumably due to mislocalization of GRK mRNA from the anterodorsal portion of the oocyte to more ventral positions. In fs(1)K10 females, expression of bunched expands ventrally, with two maxima in the anterodorsal anteroventral follicle cells, diminishing laterally. In stage 10 follicles from Egfr mutants expression of bun is lost from the dorsal anterior; reduced bun expression is shifted to more posterior follicle cells. Egg chambers from a gurken mutant completely lack dorsal appendages. No bunched expression is seen in the dorsal anterior follicle cells from stage 10 gurken mutant egg chambers. Clonal analysis shows that bun is required for the proper elaboration of dorsal cell fates leading to the formation of the dorsal appendages. Eggs from bunched mutants are shortened and their dorsal appendages are short and often wide, with branched and split ends (Dobens, 1997).

Preliminary evidence indicates the bunched is sensitive to decapentaplegic levels in the follicle cells. It is therefore thought that normal bunched expression in the dorsal anterior follicle cells is the result of combined action of the Egfr receptor for Grk and serine/threonine kinase receptors (see Thick veins and Punt) for Decapentaplegic (Dobens, 1997).

The Drosophila eggshell, which has a pair of chorionic appendages (dorsal appendages) located asymmetrically along both the anterior/posterior and dorsal/ventral axes, provides a good model to study signal instructed morphogenesis. Broad-Complex, a gene encoding zinc-finger transcription factors, is essential for the morphogenesis of dorsal appendages and is expressed in a bilaterally symmetrical pattern in the lateral-dorsal-anterior follicle cells during late oogenesis. This pattern of expression is induced and specified along the dorsoventral axis by an epidermal growth factor receptor signaling pathway, which in the oocyte includes Gurken, a localized transforming growth factor alpha-like molecule. In the surrounding somatic follicle cells, Torpedo, the Drosophila EGF receptor homolog that functions as the target of Gurken specifies BR-C expression. Mutants that result in the mislocalization of Gurken, such as fs(1)K10, induce a BR-C late expression pattern that is expanded to the ventral follicle cells surrounding the oocyte. This expanded BR-C expression results in expansion of the dorsal appendages to the ventral region. Four extra copies of grk gene increase the gap between the two groups of BR-C expressing cells to about 8 cells wide, in comparison with the 4-cell-wide gap in wild type, resulting in a widened dorsal gap between the two dorsal appendages. A decrease in Grk-Egfr signaling in a topQY1 mutation in Egfr results in BR-C expression in the dorsal most follicle cells, leading to a fusion of the dorsal appendages in the dorsal-most region. The Egfr target gene pointed regulates the number of BR-C expressing cells. Ectopic expression of pnt decreases the number of BR-C expressing cells, suggesting that Pnt regulates BR-C expression. The precisely localized expression of BR-C along the AP axis requires a separate signaling pathway, initiated by a transforming growth factor-beta homolog, Decapentaplegic, in nearby follicle cells. These two signaling pathways (Gurken functioning from the oocyte and Dpp functioning from the follicle cells) co-ordinately specify patches of follicle cells to express the Broad-Complex in a unique position with respect to both DV and AP axes respectively, and which, in turn direct the differentiation of the dorsal appendages in the correct position on the eggshell (Deng, 1997).

Brainiac functions in oogenesis in modulating the Gurken-Egfr interaction during genesis of the follicular epithelium. brainiac is expressed in germ cells at the time follicle cells first surround the nurse cell-oocyte complex. brainiac and Egfr interact in the initial formation of the follicle cells in the germarium. brainiac-Egfr double mutants lay only a few or no eggs, in contrast to single mutants whch lay hundreds of eggs. In double mutants most follicles have more than one oocyte and more than 15 nurse cells. These follicles are formed within the germarium and proceed to late stages of oogenesis. Occasionally such chambers will proceed with chorion synthesis and form an egg in the shape of a ball, lacking overt polarity. Prefollicular cells fail to migrate between each oocyte/nurse cell complex, resulting in follicles with multiple sets of oocytes and nurse cells. brn and Egfr function is also required for establishing and/or maintaining a continuous follicular epithelium around each oocyte/nurse cell complex (Goode, 1992).

The ventralizing mutations gurken, Egfr (also known as torpedo), and cornichon are all epistatic to squid. Strong alleles of grk and top can act as dominant suppressors of sqd dorsalization (Kelley, 1993).

Directed cell migration is important for many aspects of normal animal development, but little is known about how cell migrations are guided or the mechanisms by which guidance cues are translated into directed cell movement. Evidence is presented that signaling mediated by the epidermal growth factor receptor (Egfr) guides dorsal migration of border cells during Drosophila oogenesis. The transforming growth factor-alpha (TGF-alpha)-like ligand Gurken appears to serve as the guidance cue. To mediate this guidance function, Egfr signals via a pathway that is independent of Raf-MAP kinase and is specific for the Egfr receptor (Duchek, 2001a).

Border cells constitute a cluster of 6 to 10 specialized somatic follicle cells that perform a stereotypic migration during Drosophila oogenesis. At the beginning of stage 9, border cells delaminate from the anterior follicular epithelium and initiate their migration between the germline derived nurse cells, toward the oocyte. About 6 hours later, at stage 10, the border cells reach the oocyte and then migrate dorsally toward the germinal vesicle (GV). The migration of border cells is essential for female fertility; however, it is not known what guides this migration. Spatial information may be provided by the surrounding tissue in the form of cell-associated or secreted guidance cues, for example, as attractive gradients. The posterior and dorsal migration phases might be guided by separate cues, or by a single cue and a fixed migration path (Duchek, 2001a).

To identify guidance cues, the following was taken into consideration: The gradient of spatial information would be perturbed if a key attractant or repellant were uniformly overexpressed. This would be expected to cause the cells to migrate inefficiently as there would be no difference between signaling in the front and the back of the cell. To identify genes capable of perturbing border-cell migration when expressed uniformly, a modular misexpression screen was performed with the P element EPg. Expression was induced in the germline (nanosGAL4:VP16 ) and in the border cells themselves (slboGAL4). Of 8500 independent insertion lines, three showed defects in border-cell migration but no detectable morphological abnormalities in the egg chamber. In one of these, EPg35521, the single EPg element is inserted in such a way that it drives expression of the gene encoding the neuregulin-like EGFR ligand Vein. Border-cell migration is affected both when Vein is expressed in the germline tissue and when it is expressed in the border cells themselves, as might be expected of a secreted molecule (Duchek, 2001a).

To determine whether the effect on migration is specific to Vein or common to Egfr ligands, secreted forms of the TGF-alpha-like ligands Gurken and Spitz were expressed in border cells. Both affect border-cell migration, with the potent ligand secreted Spitz having the strongest effect. Border-cell expression of an activated, ligand-independent, form of Egfr [lambda-top] also severely affects migration. Thus, constitutive stimulation of Egfr signaling in border cells effectively inhibits their migration (Duchek, 2001a).

To determine whether Egfr signaling is required for normal border-cell migration, a dominant negative form of the receptor (DN-DER) or the transmembrane Egfr inhibitor Kekkon-1 was expressed in border cells. Both specific Egfr inhibitors severely inhibit dorsal migration of border cells, with only minor effects on the initial posterior migration. Most eggs from these females do not hatch and appear unfertilized. This phenotype mimics loss of border-cell function, suggesting that the dorsal aspect of migration may be essential. The requirement for Egfr in border cells was confirmed by looking at clones of Egfr mutant cells. When all outer border cells were mutant for Egfr, the cluster remained in the center of the egg chamber at stage 10, whereas 90% of wild-type clusters were found dorsally. When mixed clusters with both wild-type and mutant cells move dorsally, the wild-type cells are in the front. Thus, Egfr signaling is required specifically for dorsal border-cell migration (Duchek, 2001a).

When border cells migrate dorsally, activating ligands for Egfr are produced by the oocyte (Gurken) and, in response to Gurken, by dorsal follicle cells (Vein and Spitz). Dorsal migration still occurs when dorsal follicle cells are mutant for vein, spitz , or rhomboid, which is required for Spitz activation. Thus, although ectopic expression of Vein or activated Spitz proteins can affect border-cell guidance, neither is required for the process. Removing Egfr from patches of dorsal follicle cells, which renders them unable to activate secondary signals, also has no effect. In contrast, dorsal migration is perturbed in gurken mutants. Ovaries from grkDC/grk2b6 mutant females show a range of defects. In mildly affected egg chambers where the GV has moved anterior and dorsal, border cells complete posterior migration but fail to migrate dorsally. In stage-10 oocytes, Gurken protein is detected in a membrane-associated gradient with the highest level at the dorsal anterior over the GV. These results are most consistent with Gurken serving as the dorsal guidance cue, although contributions from other Egfr ligands cannot be excluded (Duchek, 2001a).

Next, an examination was performed to see which intracellular signaling pathways downstream of Egfr might mediate the effect on border-cell migration. Egfr signaling has been shown to regulate growth and differentiation during Drosophila development via activation of the Raf-MAP kinase (MAPK) pathway. Moderate activation of this pathway is observed in migrating border cells at both phases of migration, particularly in the leading cells. Mammalian tissue culture studies have indicated, however, that mitogenic and migration-inducing activities of Egfr and other receptor tyrosine kinases (RTKs) may occur via different pathways, prompting further investigation (Duchek, 2001a).

To investigate the role of the Raf-MAPK pathway, clonal analysis was performed with a raf null mutant (phl11). When all outer border cells are mutant, migration is normal during stage 9. Mutant clusters are very rarely recovered at stage 10, but dorsal migration can occur. Expression of an activated form of Raf (RafGOF) in border cells results in robust activation of MAPK but has no effect on border-cell migration. Finally, expression of an activated form of the Drosophila fibroblast growth factor (FGF) receptor Heartless strongly activates MAPK in border cells but has no effect on migration. This contrasts with the effect of Egfr. Thus, the effects of Egfr signaling on border-cell migration appear to be specific (not elicited by all RTKs) and independent of Raf-MAPK (Duchek, 2001a).

The small guanosine triphosphatase Ras can link RTKs to MAPK pathway or other pathways. Dominant negative Ras (RasN17) and activated Ras (RasV12) moderately affect posterior and dorsal border-cell migration, indicating that Ras has a role in both migrations. Phosphatidylinositol 3-kinase (PI3K) has been implicated directly as regulator of chemotaxis in different systems. However, expression of dominant negative or activated forms of the Drosophila PI3K catalytic subunit (p110DN and p110CAAX) does not affect border-cell migration. Phospholipase C-gamma (PLC-gamma), which can bind directly to RTKs via its SH2 domain, may mediate effects on movement of tissue culture cells. In the Drosophila genome, there appears to be only one PLC-gamma, encoded by the small wing (sl) locus. Null mutants in sl do not affect border-cell migration. Thus, neither PI3K nor PLC-gamma appear to be key mediators downstream of Egfr in this context (Duchek, 2001a).

Border cells are sensitive to Egfr signaling from the onset of migration, which suggests that the posterior migration may be guided by a similar RTK signal. Activated Heartless has no effect on migration. breathless mutant border cells migrate normally, and overexpression of the ligand Branchless has no effect. In addition, border cells mutant for dof, which is required for signal transduction by both FGF receptors, migrate normally. Thus, neither of the two Drosophila FGF receptors, Breathless and Heartless, perform this role (Duchek, 2001a).

The RTKs of the EGF receptor family are required for growth, survival, differentiation, and migration of various cell types during animal development. EGF signaling also stimulates growth and metastatic potential of human tumors, as well as proliferation and motility of tissue culture cells. These results demonstrate that Egfr signaling can direct cell migration in vivo. Egfr acts as a guidance receptor for border cells during oogenesis and is specifically required for the second phase of their migration. Another RTK with similar signaling properties may serve this function for the first phase of migration. Evidence has been presented that guidance effects of Egfr are mediated by a noncanonical signaling pathway. The challenge is now to determine which pathways and molecules downstream of Egfr translate guidance information into directed cell movement in vivo (Duchek, 2001a).

In addition to PDGF- and VEGF-receptor related (Pvr), Egfr also has properties consistent with a role in guiding border cells to the oocyte: both receptor tyrosine kinases are expressed in border cells, and their ligands are found in key locations in the germline. Both give similar gain-of-function effects, and both dominant negative receptors give subtle effects with respect to migration to the oocyte. One possible explanation for the subtle dominant negative effects is that the receptor/ligand pairs are partially redundant. This possibility was first addressed by coexpressing both dominant negative receptors in border cells. This gave a very dramatic effect. Border cells expressing both dominant negative receptors migrate very inefficiently. When quantified at stage 10, 90% of border cell clusters expressing both dominant negative receptors had migrated less than halfway to the oocyte. In 5% of egg chambers, border cell clusters were found off the direct track to the oocyte. This suggests that the cells are motile but poorly guided. This 'off track' phenotype is not observed in wild-type egg chambers or in egg chambers where border cell migration is impaired for another reason (slbo mutant) (Duchek, 2001b).

The effect of expressing dominant negative receptors in Pvf11624 mutant egg chambers was also tested. As expected, the Pvf11624 mutant phenotype is not made worse by removing activity of its cognate receptor, Pvr. However, reducing activity of the other pathway by expression of dominant negative Egfr has a strong effect. Border cells are not able to reach the oocyte by stage 10, and they also show a low level of 'off track' migration. This confirms the redundancy of function for the two receptors, as well as their ligand specificity. Thus, if either Egfr or Pvr (and corresponding ligand) are left intact, border cells can find the oocyte, but if both receptor functions are severely affected, they cannot. That Egfr is uniquely required for dorsal migration of border cells is explained by the ligand distribution. Only Egfr ligands are expressed differentially on the dorsal side. Gurken is expressed by the dorsally located germinal vesicle, and the protein is found in a gradient originating from there. Spitz and Vein are expressed in dorsal follicle cells (Duchek, 2001b).

These results indicate that Pvr and Egfr are guidance receptors for border cell migration toward the oocyte. A guidance function implies that the critical parameter for proper migration is the differential distribution of signal (ligand) rather than absolute level of signaling. This is supported by the observation that increased expression of Pvr in border cells suppresses the effect of the ectopically expressed Egfr ligand, Vein. The level of Pvr+Egfr signaling in border cells is likely higher upon coexpression, but the signal distribution might be more normal due to increased sensitivity to the spatially graded Pvr ligand relative to the ectopically expressed Egfr ligand. To test the importance of signal distribution versus level more directly, signaling from one receptor was reduced by expression of its dominant negative form and it was asked whether the deleterious effect of ectopic ligand for the other receptor would be enhanced or suppressed. For guidance signaling, the expectation is that cells which can only respond to one type of ligand will require this ligand to be properly distributed and thus be very sensitive to its misexpression. If just the correct level of signal is required, then simultaneously increasing and decreasing signaling should give a less severe phenotype than either alone. The experiment was done for both receptors, and in both cases, a strong enhancement of the migration defect was seen. Ectopic expression of one ligand and the dominant negative form of the other receptor causes a phenotype similar to one expressing both dominant negative receptors: border cells do not reach the oocyte at stage 10. They usually had migrated less than halfway, and sometimes were found off track. As expected, coexpression of a ligand with a dominant negative version of its cognate receptor has little or no additional effect. These results indicate that both receptors receive directional information which guides cell migration. Migration can proceed to some extent if only one receptor receives nonuniform (directional) signaling, consistent with a partially redundant guidance function (Duchek, 2001b).

Soma-germline interactions coordinate homeostasis and growth in the Drosophila gonad

The ability of organs such as the liver or the lymphoid system to maintain their original size or regain it after injury is well documented. However, little is known about how these organs sense that equilibrium is breached, and how they cease changing when homeostasis is reached. Similarly, it remains unclear how, during normal development, different cell types within an organ coordinate their growth. This study shows that during gonad development in Drosophila the proliferation of primordial germ cells (PGCs) and survival of the somatic intermingled cells (ICs) that contact them are coordinated by means of a feedback mechanism composed of a positive signal and a negative signal. PGCs express the EGF receptor (EGFR) ligand Spitz, which is required for IC survival. In turn, ICs inhibit PGC proliferation. Thus, homeostasis and coordination of growth between soma and germ line in the larval ovary is achieved by using a sensor of PGC numbers (EGFR-mediated survival of ICs) coupled to a correction mechanism inhibiting PGC proliferation. This feedback loop ensures that sufficient numbers of PGCs exist to fill all the stem-cell niches that form at the end of larval development. It is proposed that similar feedback mechanisms might be generally used for coordinated growth, regeneration and homeostasis (Gilboa, 2006).

Each ovary in the adult fruitfly is composed of 16-18 units called ovarioles. At the anterior of each ovariole, two or three germline stem cells (GSCs) interact with somatic cells that affect their establishment, maintenance and differentiation. These somatic niche cells develop across the larval gonad at the third larval instar and are separated into ovarioles during early pupal development. The GSCs are derived from primordial germ cells (PGCs) that form in the early embryo. During the three larval instars, the entire gonad grows. The number of PGCs increases eightfold, from about 12 PGCs in each embryonic gonad to about 100 by the middle of third instar (ML3). PGCs double their numbers every 24 h during first (L1) and second (L2) instar, and division rates are slightly slower during the next 24 h. By ML3, sufficient PGCs exist to fill all the somatic GSC niches that form at that time (Gilboa, 2006).

When embryos contain very few PGCs (either when few PGCs are transplanted into embryos lacking germ cells, or in certain genetic backgrounds), they nonetheless develop into fully fertile females. To test whether PGC proliferation might be regulated during larval growth, PGC numbers were counted in germcell-less (gcl) and oskar (osk) mutant embryos, in which fewer PGCs form during embryogenesis. In gcl mutants, in which only two PGCs on average were incorporated into the embryonic gonad, an average of about 60 PGCs was reached by ML3. This division rate (on average five divisions in three days) is higher than in the wild type (three divisions in three days). In osk mutants, in which 3 PGCs were incorporated into the gonad, the division rate was faster than in the wild type until the end of L2 (EL2), but as PGCs approached wild-type numbers, their proliferation rate decreased markedly. Thus, PGC proliferation is regulated, and can increase or decrease to achieve wild-type numbers (Gilboa, 2006).

To determine how PGC division rate is controlled, the Gal4/UAS system was used to express dominant-active or dominant-negative signalling pathway components in the larval gonad. For somatic expression the soma-specific C587-Gal4 was used. For PGC-specific expression nos-Gal4-VP16 was used. A large increase was observed in PGC numbers when the dominant-negative form of EGFR (UAS-EgfrDN) was expressed in somatic gonadal cells by C587-Gal4. Similarly, somatic overexpression of Ras85D.N17, a dominant-negative form of Ras85D, which acts downstream of EGFR, yielded an expansion of PGCs. This indicates that EGFR might function in somatic gonadal cells to regulate PGC divisions (Gilboa, 2006).

To analyse how EGFR signalling affects PGC numbers, the gonadal expression of different EGFR signalling components was determined. Consistently with the somatic effect of UAS-EgfrDN, EGFR and the phosphorylated form of mitogen-activated protein kinase (pMAPK) were detected by antibody staining in somatic cells adjacent to PGCs. Enhancer traps in both vein and argos, which are known transcriptional targets of EGFR signalling, were also expressed in these cells. Enhancer trap expression of the EGFR ligand Spitz was found in PGCs. Gurken, an additional EGFR ligand, which is expressed in oocytes, could not be detected in PGCs. Ligand presentation requires Rhomboid family members (Rhomboid or Stet) and Star in the ligand-producing cells. Both star and stet, but not rhomboid, were detected by enhancer trap expression in PGCs. These results indicate that PGCs might produce Spitz and activate EGFR signalling in neighbouring somatic cells (Gilboa, 2006).

Very little is known about the origin or function of the somatic cells adjacent to PGCs, which have been termed intermingled cells (IC). These cells express the MA33 enhancer trap at L3. ICs also express the protein Traffic Jam (TJ), although before L3 TJ expression is not limited to ICs. The results demonstrate that the EGFR signalling pathway is activated in ICs. To determine its role in ICs, a temperature-sensitive allelic combination of EGFR (Egfr(ts)) was used to overcome the earlier, embryonic, requirement for EGFR. PGCs overproliferated in Egfr(ts) larvae grown at the restrictive temperature, whereas the gonads of their wild-type siblings remained normal. Although ICs were readily observed in heterozygous gonads, fewer of them were present in Egfr(ts) gonads. A similar reduction in IC numbers was observed in UAS-EgfrDN gonads. Antibodies against cleaved caspase 3, an apoptotic marker, revealed a significant increase in dying cells in UAS-EgfrDN gonads at EL2 in comparison with those expressing C587-Gal4 alone,. Other aspects of somatic differentiation and morphogenesis seemed normal, because components of the niche such as terminal filament and cap cells differentiated normally in Egfr(ts) and UAS-EgfrDN flies. Thus, EGFR signalling is required for IC survival. A similar role for EGFR signalling in cell survival has been described in the Drosophila nervous system, in which neuronal cells secrete Spitz and protect midline glial cells from death. To determine whether IC death resulted directly from the abrogation of EGFR signalling, or indirectly from PGC overproliferation, UAS-CycD, UAS-Cdk4 were mis-expressed in PGCs. Under these conditions, PGCs overproliferated extensively, without loss of ICs. This indicates that PGC overproliferation in UAS-EgfrDN and in Egfr(ts) might have been the effect rather than the cause of IC death (Gilboa, 2006).

To ask more directly whether PGC production of Spitz is required for IC survival, Spitz production was reduced by RNA interference (UAS-spiRNAi) in either somatic gonadal cells or PGCs. Expression of UAS-spiRNAi in PGCs resulted in a significant increase in PGC numbers, whereas somatic expression had no effect. As could be expected, reducing Spitz production in PGCs resulted in reduced IC numbers. Then EGFR signalling was examined in gonads lacking PGCs altogether. At L1 and L2, pMAPK was absent, pMAPK was detected in a subpopulation of migrating cells but not where ICs are located. In gcl mutants containing PGCs, the strongest pMAPK staining was observed in somatic cells contacting PGCs. In gcl gonads lacking PGCs, ICs could be detected by TJ expression, but in greatly reduced numbers. This is not due to a general reduction in somatic cell numbers, because similar numbers of terminal filaments form in gcl and in wild-type gonads. The reduction in IC number resembles the disappearance of inner-sheath cells from adult germaria lacking germ cells. MA33 could not be detected in gcl gonads. The difference could be due to weaker staining of MA33 than that of TJ, or because MA33-positive cells represent a subpopulation of ICs that disappears in gonads lacking PGCs. Taken together, these results indicate that PGCs produce Spitz and activate EGFR signalling in ICs, which is necessary for their survival. In return, ICs inhibit PGC proliferation. It is suggested that this feedback mechanism allows the gonad to monitor and correct PGC numbers during larval growth. When very few PGCs form, Spitz production is low, leading to reduced IC numbers. This, in turn, leads to increased PGC proliferation. Compensation of PGC numbers by the end of larval development ensures that sufficient PGCs are present to occupy the adult niches (Gilboa, 2006).

To test whether EGFR signalling in ICs has an additional role to that of promoting survival, EGFR signalling was increased by mis-expressing constitutively active forms of EGFR signalling components (UAS-EgfrCA, UAS-Ras85D.G12V or UAS-phl.gof) in the soma, or mis-expressing the secreted form of Spitz in PGCs (UAS-sSpi). PGC numbers were significantly reduced in these cases. Interestingly, IC numbers remained unchanged. The restriction of TJ expression to ICs was also unchanged. Because increasing EGFR signalling resulted in decreased PGC numbers, without an apparent effect on gonad morphogenesis or IC numbers, it is suggest that EGFR signalling in ICs might be directly required for the inhibition of PGC proliferation (Gilboa, 2006).

Soma-germline interactions through EGFR signalling are a recurring motif in Drosophila. In females they serve to pattern the eggshell and localize the oocyte nucleus. In males they serve to restrict GSC proliferation and promote differentiation. The rhomboid homologue stet is required in both males and females for GSC differentiation and for proper connections between somatic cells and germ cells. The signals originating in the somatic cells and perceived by germ cells remain unknown. In this study it has been show that EGFR has a central role in a feedback loop coordinating IC survival and PGC proliferation. The properties of this loop make it ideal for regulating homeostasis and for coordinating the growth of different cell populations in any organ. In the liver, for example, several cell types proliferate after injury. It has been suggested that hepatocytes provide the mitogenic stimuli for other liver cells, such as Kupffer cells, hepatic stellar cells and biliary ductular cells. The production of transforming growth factor β by hepatic stellar cells may, in turn, limit hepatic growth. Similar feedback loops may apply in other cases during normal development or after injury (Gilboa, 2006).

PVR and EGFR signalling during collective migration of border cells

Although directed migration is a feature of both individual cells and cell groups, guided migration has been studied most extensively for single cells in simple environments. Collective guidance of cell groups remains poorly understood, despite its relevance for development and metastasis. Neural crest cells and neuronal precursors migrate as loosely organized streams of individual cells, whereas cells of the fish lateral line, Drosophila tracheal tubes and border-cell clusters migrate as more coherent groups. This study used Drosophila border cells to examine how collective guidance is performed. It is reported that border cells migrate in two phases using distinct mechanisms. Genetic analysis combined with live imaging shows that polarized cell behaviour is critical for the initial phase of migration, whereas dynamic collective behaviour dominates later. PDGF- and VEGF-related receptor and epidermal growth factor receptor act in both phases, but use different effector pathways in each. The myoblast city (Mbc, also known as DOCK180) and engulfment and cell motility (ELMO, also known as Ced-12) pathway is required for the early phase, in which guidance depends on subcellular localization of signalling within a leading cell. During the later phase, mitogen-activated protein kinase and phospholipase Cγ are used redundantly, and it was found that the cluster makes use of the difference in signal levels between cells to guide migration. Thus, information processing at the multicellular level is used to guide collective behaviour of a cell group (Bianco, 2007).

Border cells perform a well-defined, invasive and directional migration during Drosophila oogenesis. They delaminate from the follicular epithelium at the anterior end of an egg chamber and migrate posteriorly, towards the oocyte, as a compact cluster. They then migrate dorsally towards the oocyte nucleus. The border-cell cluster consists of about six outer migratory border cells and two inner polar cells that induce migratory behaviour in the outer cells but seem to be non-migratory. Two receptor tyrosine kinases (RTKs), PDGF- and VEGF-related receptor (PVR) and epidermal growth factor receptor (EGFR), are guidance receptors for border cells. Both receptors act redundantly during posterior migration towards the oocyte, whereas EGFR and its dorsally localized ligand, Gurken, are essential for dorsal migration. Localized signalling from the RTKs is important and actively maintained, especially early in migration. Rac and the atypical Rac exchange factor Mbc (myoblast city, also known as DOCK180) are important effectors. To determine the contribution of Mbc and related proteins, a loss-of-function allele of their common cofactor ELMO (engulfment and cell motility, also known as Ced-12) was generated by homologous recombination. Clusters of elmo mutant border cells arrested early in migration, a defect that could be rescued by expressing elmo complementary DNA. As for mbc, reduction in elmo function suppressed F-actin accumulation caused by constitutive PVR signalling, placing ELMO downstream of the receptor in this respect (Bianco, 2007).

To determine whether later steps in migration also depend on ELMO, mosaic border-cell clusters consisting of wild-type and mutant cells were investigated. If a mutation does not affect migration, mutant cells should be distributed randomly within the cluster. Mutant cells defective in migration would be in the rear, 'carried along' by normal cells. As expected, Pvr and Egfr double mutant cells were in the rear during posterior migration, as were Egfr mutant cells during dorsal migration, reflecting the requirements at each stage. elmo mutant cells were in the rear during the initial migration, but were equally frequent in the leading position during dorsal migration. This indicates that, although ELMO is essential for the early-phase signalling, the later phase of migration does not require the Mbc-ELMO complex (Bianco, 2007).

To understand late guidance signalling, EGFR signalling, on which dorsal migration depends, was dissected. Uniformly activated EGFR, like PVR, dominantly impairs migration. The carboxy-terminal tail of EGFR was essential for this activity. Systematic mutagenesis of all docking tyrosines to phenylalanine identified Y1357 as being critical, with minor contributions from Y1405 and Y1406. Other tyrosines, including Y1095 in the conserved activation loop (phosphorylated in HER2 (Human EGF Receptor 2), were not required. Twenty Src-homology 2- and phosphotyrosine-binding-containing signalling molecules were tested for binding to active EGFR and tyrosine mutants. Y1357 was necessary and sufficient for binding of the adaptor protein Shc and its phosphotyrosine-binding domain. No other tested interactor behaved in this way. Binding was confirmed by immunoprecipitation. Border cells mutant for Shc showed no dorsal migration and, when PVR signalling was also blocked, these cells showed severely impaired posterior migration. This phenotype is identical to that of Egfr mutant cells, suggesting that Shc is essential immediately downstream of EGFR for guidance signalling (Bianco, 2007).

The Shc adaptor protein links EGFR and other RTKs to mitogen-activated protein kinase (MAPK) kinase signalling as well as to other classical downstream pathways. Raf, phospholipase Cγ (PLC-γ) or phosphatidylinositol-3-OH-kinase are not uniquely required for migration; however, the pathways might act redundantly. Simultaneous perturbation of PLC-γ and Raf impaired migration, with no effect of phosphatidylinositol-3-OH kinase. Double mutant border-cell clusters, cell-autonomously lacking PLC-γ and Raf or lacking PLC-γ and MAPK kinase (MAPKK), initiated migration but were delayed later in posterior migration and showed no dorsal migration. This phenotype is more severe than that of Egfr or Shc alone, suggesting that both RTKs might be affected. Prevention of PVR activity in double mutant cells did not block posterior migration, confirming that the requirement for these pathways was stage-specific and not EGFR-specific. Finally, analysis of mosaic clusters showed that Raf/MAPK and PLC-γ were important in late migration, reciprocal to the requirement for elmo. These results genetically define two migratory phases: an early posterior phase requiring ELMO-Mbc and a later posterior and dorsally directed phase requiring Raf/MAPK or PLC-γ. Both RTKs shift effector-pathway-dependency as migration progresses (Bianco, 2007).

To investigate the different migratory phases, border-cell migration was examined via live imaging. Appropriate conditions were establised for culturing and imaging of egg chambers, considering only active, growing ones. Border cells were selectively labelled with green fluorescent protein (GFP) and all membranes were labelled with the vital dye FM4-64. For all 24 wild-type samples, the identity of the front cell changed during the observation period, confirming the inference from fixed samples that cells change position during migration. This indicates that there is no determined front-cell fate. A clear difference was observed in behaviour of clusters during early (first half) and late phases. Early clusters had one, sometimes two, highly polarized cells clearly leading the migration; once these cells delaminated they moved straight and relatively fast. Weakly stained extensions protrude far from delaminating cells and subsequently shorten during movement, suggesting a 'grapple and pull' mechanism. Midway towards the oocyte, strong polarization was lost and cells rounded and started to 'shuffle' while dynamically probing the environment with short extensions. Occasionally the cluster would rotate or 'tumble' completely. This shuffling behaviour still provided effective movement of the cluster towards the oocyte and dorsally, albeit more slowly. Labelling cells with nuclear GFP allowed visualization of changes in positions within the cluster. The front cell exchanged, on average, every 18 min (Bianco, 2007).

As expected, positions corresponding to the second, slower phase of migration were more represented when cluster position along the migratory path was quantified in fixed samples. Also, border cells expressing dominant negative PVR and EGFR were individually active but provided little net cluster movement, as expected from the lack of guidance information. Finally, uniform overexpression of the attractant PVF1 caused an increased shuffling behaviour in the early phase but allowed slow forward movement, resembling normal late migration. This indicates that migrating clusters can interpret a shallow gradient when using the shuffling mode. It also suggests that the normal change in migratory behaviour midway into posterior migration might be triggered by the higher concentration of ligands closer to the oocyte (Bianco, 2007).

The early phase of migration with a highly polarized front cell corresponds temporally to the genetic requirement for ELMO activity. During the later phase, individual elmo mutant cells can alternate with wild-type cells in the lead position. Genetic analysis showed that Raf and MAPKK and, by inference, MAPK activation was sufficient to convey late guidance information. This was puzzling because MAPK activation appeared uniform in migrating border cells, and localized effects are usually a hallmark of guidance signalling. However, signalling that is not localized within an individual cell could still transmit spatial guidance information to the cell cluster if the cell with higher overall signalling indicates the direction of subsequent migration for the whole cluster, as observed for MAPK signalling in border cells. In this 'collective guidance' scenario, each cell of the cluster can be thought of as being analogous to a sector of an individual guided cell. Different levels of signalling in individual cells of the cluster transform into migration vectors because border cells adhere to each other and these contacts differ from substrate contacts. The occasional tumbling of border-cell clusters emphasizes the ability of these cells to behave as a collective unit. Tumbling may help single guided cells to 'reassess' their environment (Bianco, 2007).

To test this model for guidance, the relative levels of signalling in individual cells of the cluster were manipulated. Dynamic shuffling should allow cells to constantly 'compete' for the front position. None of the manipulations discussed below improved migration if all cells in a cluster were affected. Individual border cells with moderately elevated levels of PVR or EGFR were preferentially in the front relative to wild-type cells. Cells with elevated PVR tended to stay in or near the front position, suggesting that they were not competed away by other cells. This bias was ligand-dependent, because reducing PVF1 levels shifted the bias from PVR to EGFR, as was also shown by analysis of dorsal migration. Thus, increased signalling gives a cell-front bias when measuring an informative ligand. Elevating intracellular signalling levels had similar effects, whether by overexpression of an active form of Raf or by preventing downregulation of signalling as in Hrs mutant cells, in which RTK-mediated MAPK signalling is elevated in enlarged endosomes. The more modest front bias in Hrs mutant cells was reflected in behaviour: they could be displaced from the front. The E3 ubiquitin ligase Cbl negatively regulates RTK signalling and is also required to maintain localized RTK signalling within border cells initiating migration. Cbl mutant cells shifted from being preferentially at the back during early stages to being in the front during later migration. This indicates a transition from a mode requiring Cbl-dependent localization of signalling within the leading cell to a mode based on collective decisions within the cluster, in which Cbl mutant cells have an advantage owing to elevated RTK signalling (Bianco, 2007).

It is suggested that guidance of border-cell migration is achieved by two means: signalling localized within the cell, as used in individual migrating cells, and collective guidance, whereby the cluster uses differences in signalling strength among its constituent cells to determine direction. The two modes use the same guidance cues and receptors, but different downstream effectors. Localized signalling is required for the initial, polarized rapid migration, whereas collective behaviour, though observable throughout, dominates in the later phase. Collective decisions on the basis of differences in RTK signalling strength are important in Caenorhabditis elegans vulval development and in branching of Drosophila tracheal tubes, in which they result in specification of discrete cell fates. This differs from the dynamic situation reported in this study, in which the identity of the leading cell constantly changes. Indeed, the frequent exchange of leading cells suggests that front behaviour is normally temporarily restricted, possibly by induced inactivation of signalling. Such dynamics may allow the cluster to better reassess the environment. For guided migration of cell groups, this analysis indicates that sensing and regulation happens both at the single cell level and at the next level-that of collective cell decisions (Bianco, 2007).

Stem cell tumor/Rhomboid-2 functions upstream of Egfr in oogenesis and spermatogenesis

The Drosophila rhomboid (rho) gene participates in localized activation of EGF-receptor signaling in various developmental settings. The Rhomboid protein has been proposed to promote presentation and/or processing of the membrane-bound Spitz (mSpi) EGF-related ligand to generate an active diffusible form of the ligand. The rhomboid-related gene, variously known as stem cell tumor (stet), brother of rhomboid (brho) and rhomboid-2 (rho-2), was first identified as a gene involved in the determination of germ line cell fate. Later, sequence similarity searches identified stet/brho/rho-2 as a rhomboid related gene. In contrast to rho, which is expressed in complex patterns during many stages of development, stet/brho/rho-2 appears to be expressed only in germ cells. brho transcripts are present in early oocytes and abut posterior follicle cells which exhibit high levels of MAPK activation. brho, like rho, collaborates with Star to promote signaling through the Egfr/Mapk pathway, and genetic evidence indicates that Brho can activate both the mSpi and the Grk precursor EGF ligands in the wing. It is proposed that endogenous brho may activate the oocyte-specific Gurken ligand and thereby participate in defining posterior cell fates in the early follicular epithelium (Schulz, 2002 and references therein).

Signaling via the Epidermal growth factor receptor (Egfr) mediates many cell-cell interactions where one cell influences the proliferation or differentiation of a closely apposed partner. Despite the exquisitely localized and temporally specific requirements for Egfr activation in normal development documented in Drosophila, the Egfr and its major ligand spitz (spi) are widely expressed. Spatial and temporal control of Egfr pathway activation appear to be achieved at the level of ligand activation. spi is synthesized as a transmembrane protein. Proteolytic cleavage of spi by the transmembrane protein Rhomboid (Rho) within the Golgi apparatus of the signal sending cell produces a potent diffusible ligand. Expression of rho is spatially and temporally controlled, providing developmental specificity to activation of the Egfr pathway (Schulz, 2002 and references therein).

In Drosophila oogenesis, germ cells signal via the germline Egfr ligand gurken (grk) to specify the correct behavior of follicle cells in encapsulating each individual cluster of 16 germ cells, and later to pattern the follicle cell layer. So far it has been unclear how Egfr is activated during oogenesis. Germline clones mutant for rho produce wild-type eggs, suggesting that rho is not required in germ cells. Instead, rho is expressed in follicle cells depending on Egfr activation, most likely to spread and amplify the initial signaling event (Schulz, 2002).

stet plays a crucial role in signaling from germ cells to somatic cells. Wild-type function of stet is required for encapsulation of germline stem cells and their progeny by somatic support cells and germ cell differentiation in both Drosophila males and females. Clonal analysis and rescue experiments in testes have demonstrated that stet function is required in germ cells. The conserved protease motif in the Stet protein and its subcellular localization suggest that stet functions through the same biochemical mechanism as rho. In support of this, expression of rho in germ cells rescues the stet mutant testes phenotype. It is proposed that stet activates signaling from germ cells to the Egfr on somatic support cells to set up the crucial associations between germ cells and soma that are required for normal gamete differentiation (Schulz, 2002).

Expression of rho in germ cells of stet mutant testes also restores spermatogenesis, indicating that stet and rho may function through the same biochemical mechanism. To explore how stet function in early male germ cells might relate to the Egfr signal transduction pathway, the expression and effects of other components of the pathway on early male germ cell differentiation were tested. Expression of secreted forms of the Egfr ligands spi and grk in male germ cells under the control of the nos-GAL4 activator did not modify the stet mutant phenotype, raising the possibility that another ligand may play a role in male germ cells. rho normally acts synergistically with the transmembrane protein Star within the signaling cell to activate spi. In situ hybridization with a Star mRNA probe to wild-type testes revealed high levels of Star expression at the apical tip (Schulz, 2002).

Consistent with a potential role for the Egfr in somatic cells, activated MAP-kinase is detectable in somatic cyst cells of wild-type testes. In stet mutant testes, MAP-kinase expression is restricted to the somatic hub cells and a few (two to five) cells next to the somatic hub in the position corresponding to that of cyst progenitor cells. Although cyst cells were present in stet mutant testes, no activated MAP-kinase was detected in cyst cells displaced away from the hub. Likewise, activated MAP-kinase is detected in the cytoplasmic extensions of inner sheath cells of wild-type germaria, but only in a few inner sheath cells in germaria from stet mutant females (Schulz, 2002).

It is concluded that the stet gene plays a crucial role in germ cell differentiation in both males and females. In animals that lack stet function, somatic support cells fail to surround germ cells properly and germ cells accumulate at early stages of differentiation. Mosaic analysis in testes suggested that stet function is required in germ cells for normal association between early germ cells and somatic cyst cells. This, along with the molecular identity of the stet gene, suggests that stet activates signaling from germ cells to the soma to allow normal interactions between germ cells and somatic support cells (Schulz, 2002).

The stet gene encodes a homolog of rho, which plays an essential role in Egfr signaling. Rho has been shown to localize to the Golgi apparatus, where it acts as a protease to cleave the Egfr ligand spi. The stet protein also localizes to the Golgi apparatus in cell culture experiments, and contains the protease motif described for Rho. Consistent with the idea that stet may encode a protease, three strong stet alleles alter residues in the conserved protease domain (Schulz, 2002).

Experiments in wing discs demonstrate that stet is able to collaborate with Star to promote signaling through the Egfr/MAP-kinase pathway. stet can activate the Egfr ligands spi and grk when ectopically expressed in wing discs or follicle cells (Guichard, 2000; Ghiglione, 2002). Expression of rho in germ cells can substitute for stet function, and Star is expressed at the tip of wild-type testes. In addition, in stet mutant testes most somatic cyst cells fail to express activated MAP-kinase, the downstream indicator for Egfr signaling. Based on these observations, it is proposed that the stet gene functions as an activator of signaling from early germ cells to the Egfr presented on the surface of somatic cells and that activation of the Egfr in somatic cells is required to establish normal interactions between germ cells and somatic cells (Schulz, 2002).

Testes from animals carrying a temperature-sensitive allele of the Egfr show accumulation of germ cells that appeared to be stem cells, gonialblasts and spermatogonia. This similarity to the stet mutant phenotype in testes is consistent with stet and the Egfr functioning in the same pathway. However, the Egfrts mutant phenotype in testes does not exactly resemble the stet mutant phenotype. In stet mutant testes, somatic cyst cells do not envelope clusters of early germ cells properly. Testes from Egfrts mutant animals display many defects in the association of somatic cyst cells and early germ cells, including some cases in which germ cell clusters were associated with multiple somatic cyst cells. Since analysis of the Egfrts phenotype was performed after a temperature shift, it is hypothesized that testes from Egfrts animals may have had sufficient residual Egfr activity to allow some and possibly abnormal association of early germ cells and somatic cyst cells. In addition, the Egfrts mutant may not be null for Egfr function at 29°C, the temperature assayed. Consistent with this likely possibility, no cyst cell clones mutant for Egfr null alleles have not been recovered, even though somatic cyst cells are detected in the Egfrts allele (Kiger, 2000). In contrast, in this study, the phenotype of animals null mutant for stet throughout development was reported (Schulz, 2002).

stet may activate a yet unidentified Egfr ligand to recruit somatic cells for germ cell encapsulation. Even though stet can activate spi and grk when ectopically co-expressed in wing discs, expression of secreted forms of spi or grk in male germ cells did not rescue the stet mutant phenotype in this study. Loss-of-function alleles of grk that cause severe defects in female gametogenesis do not show an early germ cell over-proliferation phenotype in testes, suggesting that stet does not function through grk activation. In females, eggs laid by stet mutant mothers do not display the grk or spi mutant phenotypes, but instead, they either show a variety of defects or develop into phenotypically normal adults. Further investigation of the Egfr signal transduction pathway remains to be undertaken to identify additional components of the pathway and test their potential role in interactions between early germ cells and surrounding somatic cells (Schulz, 2002).

The data predict a new function for the Egfr signaling pathway in the female gonad. Egfr signaling, activated by stet, may be required to set up the normal interactions of early female germ cells and somatic inner sheath cells in region 1 and 2A of the germarium. No accumulation of early germ cells with cytoplasmic Sxl protein and spectrosomes was observed at the tip of the germarium after shifting animals carrying the Egfrts allele to the restrictive temperature as adults. However, many ovarioles from females carrying Egfrts alleles also did not display defects at later stages of oogenesis in these experiments, again indicating that the Egfrts allele has residual Egfr activity and may not reflect the Egfr loss-of-function situation (Schulz, 2002).

The possibility that stet activates other signaling pathways to set up proper physical interactions between germ cells and somatic support cells cannot be ruled out. In females, normal encapsulation of germ cells by somatic follicle cells requires the neurogenic genes brainiac (brn) and egghead (egh). brn encodes a secreted protein and egh encodes a secreted or transmembrane protein. Double mutant combinations of grk and brn display much stronger defects in encapsulation of germ cells than either grk or brn mutants alone, suggesting that the brn and egh pathway and the Egfr pathway function partially redundantly in formation of the prefollicular epithelium. This opens the possibility that encapsulation of early female germ cells by inner sheath cells and encapsulation of male germ cells by somatic cyst cells depend on another signaling pathway instead of, or in addition to, the Egfr signal transduction pathway (Schulz, 2002).

Control of germline stem cell division frequency--a novel, developmentally regulated role for epidermal growth factor signaling

Exploring adult stem cell dynamics in normal and disease states is crucial to both better understanding their in vivo role and better realizing their therapeutic potential. This study addresses the division frequency of Germline Stem Cells (GSCs) in testes of Drosophila melanogaster. GSC division frequency is under genetic control of the highly conserved Epidermal Growth Factor (EGF) signaling pathway. When EGF signaling was attenuated, a two-fold increase in the percentage of GSCs in mitotic division was detected compared to GSCs in control animals. Ex vivo and in vivo experiments using a marker for cells in S-phase of the cell cycle showed that the GSCs in EGF mutant testes divide faster than GSCs in control testes. The increased mitotic activity of GSCs in EGF mutants was rescued by restoring EGF signaling in the GSCs, and reproduced in testes from animals with soma-depleted EGF-Receptor (EGFR). Interestingly, EGF attenuation specifically increased the GSC division frequency in adult testes, but not in larval testes. Furthermore, GSCs in testes with tumors resulting from the perturbation of other conserved signaling pathways divided at normal frequencies. It is concluded that EGF signaling from the GSCs to the CySCs normally regulates GSC division frequency. The EGF signaling pathway is bifurcated and acts differently in adult compared to larval testes. In addition, regulation of GSC division frequency is a specific role for EGF signaling as it is not affected in all tumor models. These data advance understanding concerning stem cell dynamics in normal tissues and in a tumor model (Parrott, 2012).

This study reports on the division dynamics of GSCs in response to attenuated EGF signaling. GSCs in EGF mutant testes contained more cells in M-phase and in S-phase of the cell cycle and it took significantly less time for all GSCs within one testes to complete one round of the cell cycle compared to GSCs in control testes. Confirming the role for EGF signaling in regulating the frequency of GSC divisions, germline-specific expression of EGF ligand rescued the hyperproliferation of GSCs in EGF mutant animals. Mutations in stet as well as RNAi-mediated knockdown of the EGFR in cyst cells recapitulated the increased mitotic activity of GSCs. These data demonstrate a novel and specific role for EGF signaling: the repression of GSC division frequency (Parrott, 2012).

Based on these findings, a model is proposed that demonstrates the bifurcation of the EGF signaling pathway (see A model depicting the requirement for EGF signaling). EGF acts in the CySCs in one pathway to regulate GSC division frequency and in a different pathway in cyst cells to promote germ cell enclosure and differentiation. This developmental bifurcation of EGF function during Drosophila spermatogenesis reveals a fundamental uncoupling between the control of stem cell proliferation and the control of stem cell daughter differentiation. The stage-specific requirement for EGF may reflect the different functions of GSCs in immature versus mature tissues. The initial function of GSCs may be to quickly populate larval testes with germline cells, while GSCs in adult testes need to replenish differentiated cells dependent on demand (Parrott, 2012).

This study is the first report of a stage-specific impact of a signaling pathway on the activity of GSCs and suggests that this developmental switch in GSC activity between larval and adult stages requires the activities of stage-specific pathways. On a molecular level, additional pathways may be active during larval stages that counteract the increased division frequency observed in adult GSCs upon loss of EGF. In larval testes, nutrient availability and cell growth may be the primary factors governing the frequency of GSC divisions. Conversely, soon after eclosion, Drosophila males reach sexual maturity and spermatogenesis may rely on EGF-mediated signaling to regulate GSC divisions (Parrott, 2012).

Qian, Y., Dominado, N., Zoller, R., Ng, C., Kudyba, K., Siddall, N. A., Hime, G. G. and Schulz, C. (2014). Ecdysone signaling opposes epidermal growth factor signaling in regulating cyst differentiation in the male gonad of Drosophila melanogaster. Dev Biol 394(2):217-27. PubMed ID: 25169192

Ecdysone signaling opposes epidermal growth factor signaling in regulating cyst differentiation in the male gonad of Drosophila melanogaster

The development of stem cell daughters into the differentiated state normally requires a cascade of proliferation and differentiation steps that are typically regulated by external signals. The germline cells of most animals, in specific, are associated with somatic support cells and depend on them for normal development. In the male gonad of Drosophila melanogaster, germline cells are completely enclosed by cytoplasmic extensions of somatic cyst cells, and these cysts form a functional unit. Signaling from the germline to the cyst cells via the Epidermal Growth Factor Receptor (EGFR) is required for germline enclosure and has been proposed to provide a temporal signature promoting early steps of differentiation. A temperature-sensitive allele of the EGFR ligand Spitz (Spi) provides a powerful tool for probing the function of the EGRF pathway in this context and for identifying other pathways regulating cyst differentiation via genetic interaction studies. Using this tool, this study showed that signaling via the Ecdysone Receptor (EcR), a known regulator of developmental timing during larval and pupal development, opposes EGF signaling in testes. In spi mutant animals, reducing either Ecdysone synthesis or the expression of Ecdysone signal transducers or targets in the cyst cells resulted in a rescue of cyst formation and cyst differentiation. Despite of this striking effect in the spi mutant background and the expression of EcR signaling components within the cyst cells, activity of the EcR pathway appears to be dispensable in a wildtype background. It is proposed that EcR signaling modulates the effects of EGFR signaling by promoting an undifferentiated state in early stage cyst cells (Qian, 2014).

Identification and analysis of a glutamatergic local interneuron lineage in the adult Drosophila olfactory system

The antennal lobe of Drosophila is perhaps one of the best understood neural circuits, because of its well-described anatomical and functional organization and ease of genetic manipulation. Olfactory lobe interneurons - key elements of information processing in this network - are thought to be generated by three identified central brain neuroblasts, all of which generate projection neurons. One of these neuroblasts, located lateral to the antennal lobe, also gives rise to a population of local interneurons, which can either be inhibitory (GABAergic) or excitatory (cholinergic). Recent studies of local interneuron number and diversity suggest that additional populations of this class of neurons exist in the antennal lobe. This implies that other, as yet unidentified, neuroblast lineages may contribute a substantial number of local interneurons to the olfactory circuitry of the antennal lobe. This study identified and characterized a novel glutamatergic local interneuron lineage in the Drosophila antennal lobe. MARCM (mosaic analysis with a repressible cell marker) and dual-MARCM clonal analysis techniques to identify this novel lineage unambiguously, and to characterize interneurons contained in the lineage in terms of structure, neurotransmitter identity, and development. The glutamatergic nature of these interneurons was demonstrated by immunohistochemistry and an enhancer-trap strain was used that reports the expression of the Drosophila vesicular glutamate transporter (DVGLUT). The neuroanatomical features of these local interneurons at single-cell resolution, and the marked diversity in their antennal lobe glomerular innervation patterns was documented. Finally, the development of these dLim-1 and Cut positive interneurons was tracked during larval and pupal stages. This study has identified a novel neuroblast lineage that generates neurons in the antennal lobe of Drosophila. This lineage is remarkably homogeneous in three respects. All of the progeny are local interneurons, which are uniform in their glutamatergic neurotransmitter identity, and form oligoglomerular or multiglomerular innervations within the antennal lobe. The identification of this novel lineage and the elucidation of the innervation patterns of its local interneurons (at single cell resolution) provides a comprehensive cellular framework for emerging studies on the formation and function of potentially excitatory local interactions in the circuitry of the Drosophila antennal lobe (Das, 2011).

Use-dependent plasticity in clock neurons regulates sleep need in Drosophila: Interactions between bs and Egfr regulate sleep responses to social experience

Sleep is important for memory consolidation and is responsive to waking experience. Clock circuitry is uniquely positioned to coordinate interactions between processes underlying memory and sleep need. Flies increase sleep both after exposure to an enriched social environment and after protocols that induce long-term memory. This study found that flies mutant for rutabaga, period, and blistered were deficient for experience-dependent increases in sleep. Rescue of each of these genes within the ventral lateral neurons (LNVs) restores increased sleep after social enrichment. Social experiences that induce increased sleep were associated with an increase in the number of synaptic terminals in the LNV projections into the medulla. The number of synaptic terminals was reduced during sleep and this decline was prevented by sleep deprivation (Donlea, 2009).

Although sleep is a process that is necessary for survival, the functions of sleep are unknown. Sleep is regulated by circadian influences and is important for consolidation of long-term memory (LTM). Additionally, LTM is modulated by circadian mechanisms. Because the relationship between sleep, memory, and circadian rhythms seem to be phylogenetically conserved, Drosophila can be used to explain mechanisms that coordinate these processes. Drosophila show an increase in daytime sleep after exposure to socially enriched environments. Similarly, an increase in sleep after courtship conditioning is necessary for LTM (Donlea, 2009).

Increased sleep after social enrichment is dependent upon genes that are required for learning and memory, including genes that alter cyclic adenosine monophosphate signaling. Although newly eclosed flies that are mutant for the adenylyl cyclase rutabaga (rut2080) show increased sleep after social enrichment, 3 to 4 day-old adult rut mutants do not respond to changes in the social environment. Elevating wild-type rut in adult flies with an RU486-inducible driver rescued experience-dependent increases in sleep in adult rut mutants; vehicle-treated siblings showed no increase in sleep. To identify circuits that mediate experience-dependent increases in sleep, a series of GAL4 lines was used to drive wild-type rut expression in brain circuits. Expression of UAS-rut using pdf-GAL4 restored the increase in daytime sleep and daytime sleep-bout duration, although to a lesser extent than GSelav. The expression pattern of pdf-GAL4 is limited to the ventral lateral neurons (LNVs), a group of clock neurons that express pigment-dispersing factor (pdf). Although pdf is the only known output from the LNVs, flies mutant for pdf show a wild-type increase in sleep (Donlea, 2009).

Given this role of clock cells, the clock gene period (per), which is expressed in the LNVs and is required for LTM, was examined. Rescue of wild-type per using a 7.2-kb fragment of the per genomic sequence (per01; per+7.2-2) restored expression of PER at CT0 within the LNVs as well as the dorsal lateral neurons, LNDs; mutant flies carrying a null mutation, per01, expressed no PER. Although per01 mutants showed no increase in sleep after social enrichment, per01;per+7.2-2 flies displayed normal experience-dependent increases in sleep. per01 mutants have no LTM when tested 48 hours after training and only show a transient increase in sleep. per01;per+7.2-2 flies displayed LTM and increases in sleep. Although per levels are low in mutants for Clock and cycle, both acquire LTM and increase sleep after social enrichment. Thus, only a very small amount of per may be required to support increased sleep and LTM (Donlea, 2009).

To further investigate the role of synaptic plasticity in clock cells, the Drosophila homolog for serum response factor (SRF), blistered (bs), was used. In mice, SRF is essential for activity-induced gene expression and plays an important role in synaptic long-term potentiation (Ramanan, 2005) and in contextual habituation (Etkin, 2006). bs retains a 93% identity with SRF within the DNA-binding MCM1-ARG80-Agamous-Deficiens-SRF (MADS) domain. Social enrichment elevated the transcription of bs in wild-type Canton-S (Cs) flies. Mutants carrying a P element inserted into the bs gene (P{GAL4}bs1348) do not increase sleep after social enrichment. This deficit was also found in flies carrying either of two other mutant alleles for bs (bs2 and bs3) and was present in flies that are homozygous for mutant bs alleles and flies that have been outcrossed to either Cs or to flies carrying the In(2LR)Px4 deficiency. The P-element insertion in bs1348 preserves the MADS domain; similar N-terminal truncated mutant SRF acts as dominant negative. BS is expressed throughout the brain, including pdf-expressing LNVs. When UAS-egfp was driven by P{GAL4}bs1348, expression was restricted to a small number of neurons, including the LNVs. Expression of bs using P{GAL4}bs1348 to drive either of two wild-type bs (UAS-bs) constructs rescued experience-dependent increases in sleep. Moreover, inducing bs expression within the LNVsusing pdf-GAL4 increased sleep after social enrichment (Donlea, 2009).

To establish whether expression of bs is required for LTM, flies carrying the P{GAL4}bs1348 mutant allele were tested using courtship conditioning. Although P{GAL4}bs1348/+ flies acquire short-term memory, LTM was impaired. Rescue of wild-type bs using P{GAL4}bs1348 restored LTM. Next, the GAL4 repressor cry-GAL80 was used to block UAS-bs expression within the LNs. Although UAS-bs/+;cry-gal80/+ control flies showed significant courtship suppression, P{GAL4}bs1348/UAS-bs;cry-GAL80/+ flies had no LTM, which suggests a role for the LNs, although a role for the dorsal neurons (DNs) cannot be excluded. Although SRF deletion in mouse forebrain results in neurons with abnormal morphology (Knoll, 2006), the morphology of LNVs in mutant P{GAL4}bs1348/+ flies did not differ from that of LNVs in P{GAL4}bs1348/UAS-bs rescue flies. All three mutants for bs had intact circadian rhythms and showed anticipatory activity before light-dark transitions; only bs3 flies show an altered period under constant darkness. These findings suggest that there are no developmental abnormalities in the LNVs in bs mutants (Donlea, 2009).

Hypomorphic alleles for bs prevent proper wing development through interactions with Epidermal growth factor receptor (Egfr) signaling. Because Egfr alters sleep in Drosophila (Foltenyi, 2007), interactions between bs and Egfr may regulate responses to social experience. After social enrichment, transcription of Egfr was significantly elevated in Cs flies. The Egfr genomic sequence contains several CC(A/T)6GG CArG elements that can be bound by bs to promote transcription, and transcription of Egfr was significantly reduced in bs mutants. Thus, P{GAL4}bs1348 was used to drive expression of a constitutively active Egfr construct (UAS-Egfr*). Although P{GAL4}bs1348/+ mutants showed no change in sleep after social enrichment, activation of Egfr in P{GAL4}bs1348/+;UAS-Egfr*/+ flies increased sleep. Conversely, the expression of a dominant-negative construct for Egfr (UAS-EgfrDN) using pdf-GAL4 prevented increases in sleep after social enrichment (Donlea, 2009).

A recent theory proposes that a function of sleep is to downscale synaptic connections. Moreover, structural plasticity can be induced by environmental manipulation in Drosophila. To quantify the effect of social enrichment on the number of post-synaptic terminals in LNV projections, pdf-GAL4 was used to drive expression of a green fluorescent protein (GFP)-tagged construct of the postsynaptic protein discs-large (UAS-dlgWT-gfp). After 5 days of social enrichment, LNV projections into the medulla of pdf-GAL4/+;;UAS-dlgWT-gfp/+ flies contained significantly more GFP-positive terminals. Although it has not been demonstrated that the labeled synaptic terminals are functional, these tools have been used to quantify synapses. The expression of the UAS-dlgWT-GFP marker did not alter synaptic function in a wild-type background and did not prevent the increase in sleep when expressed using pdf-GAL4 after social enrichment. To determine the effect of waking on synapse number, socially isolated pdf-GAL4/+;;UAS-dlgWT-gfp/+ flies and their enriched siblings either were allowed to sleep ad libitum or were sleep deprived for 48 hours after social enrichment. Although the number of dlg-GFP positive terminals remained elevated in sleep-deprived socially enriched flies, terminal number was significantly reduced in siblings that were allowed to sleep. Similarly, the number of presynaptic terminals in LNV projections into the medulla using a GFP-tagged construct of the presynaptic protein synaptobrevin (UAS-VAMP-GFP) in pdf-GAL4/+;UAS-VAMP-GFP/+ flies was increased. After 48 hours of recovery, socially enriched pdf-GAL4/+;UAS-VAMP-GFP/+ flies had a reduced number of VAMP-GFP-positive presynaptic terminals relative to their sleep-deprived siblings. A recent study has reported a clock-dependent remodeling in the axonal terminals of the PDF circuit that is highest during the day. Recent data indicates that hyperexcitation of a subset of the LNVs suppresses sleep in Drosophila. Together with the current results, these data suggest that the PDF circuit is well suited to test the hypothesis that sleep acts to downscale synaptic connections that are potentiated during waking experience (Donlea, 2009).

Table of contents

EGF receptor : Biological Overview | Evolutionary Homologs | Regulation | Protein Interactions | Developmental Biology | References

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