Effects of Mutation or Deletion

Table of contents

Notch and oogenesis

hedgehog is expressed in the extreme apical end of fly ovarioles and in terminal filament cells and associated somatic cells. hh activity stimulates the proliferation of pre-follicle somatic cells, and promotes the specification of polar follicle cells. hh signaling during egg chamber assembly appears to be closely related to, or part of pathways involving the neurogenic genes. Egg chamber production involves the specification of several somatic cell types, including polar cells and stalk cells. Polar cells, somatic cells usually present at both the anterior and posterior poles of the oocyte, appear ectopically throughout egg chambers exposed to elevated levels of HH during their formation. Reduced activity of Notch and Delta also causes the production of an increased number of polar cells at the ends of egg chambers as well as the loss of stalk cell fate. hh signaling specifies the proper anterior-posterior orientation of polar cells, while cell-cell interactions mediated by N and DL ensure that only two cells maintain this fate prior to egg chamber formation in Drosophila (Forbes, 1996).

In wild type germarium, egg chambers are formed by the inter-leaving of follicle cells between adjacent germ-line clusters, and subsequently, a small population of specialized follicle cells, called stalk cells, form a narrow stem between adjacent hambers. In germaria mutant for Notch and Delta both these processes are defective: Follicle cells do not properly interleaf between adjacent clusters, and stalk cells are morphologically abnormal and fail to form a stalk. In mutant somatic cells there is an overabundance of polar follicle cells, which normally are present as a pair of specialized cells in the terminal follicle cell cluster. The Notch system may function in the germarium to sort main body follicle cells, terminal follicle cells and stalk cells, and then, in the vitellarium to sort terminal follicle cells and polar follicle cells (Ray, 1996).

The roles of Notch were studied in ovarian follicle cells during Drosophila oogenesis. These studies show directly that constitutively active Notch arrests cells at a precursor stage, while the loss of Notch function eliminates this stage. Expression of constitutively active Notch in the germarium follicle cells generates long stalk structures. Expression of moderate levels of activated Notch leads to partial transformation of cell fates, and this milder phenotype correlates with a prolonged, but still transient, precursor stage. This prolongation is detected by examining two markers for differentiated cells, Big brain and FasIII. In long stalks, more cells were detected that stain with a Bib precursor pattern and early Fas III pattern than in wild type ovaries, indicating that more precursors are present in these ovarioles. However, if these stalks are allowed to develop, the cells mature (or partially mature) to stalk cells or pole cell. Expression of constitutively active Notch in follicle cells at later stages leads to a defect in the anterior-posterior axis of the oocyte. Constitutively active Notch induces premature ooplasmic streaming, suggesting a replacement of the normal gradient of microtubules by subcortical microtubule bundles (subcortical bundles are associated with ooplasmic streaming). Exactly how Notch alters microtubule distribution is unknown (Larkin, 1996).

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. Notch is shown to be required for the correct subdivision of the terminal follicle cells. Although the phenotype of Notch and Delta mutants provided the first evidence that the posterior follicle cells play a role in the polarization of the oocyte, it is still not known at which step in anterior-posterior axis formation Delta/Notch signaling is required. To address this question, an examination was performed to determine whether the N ts mutant disrupts this pathway before or after the induction of the posterior follicle cells by the oocyte. Since the most sensitive assay for a failure in posterior follicle cell determination is transformation to anterior fate, the expression of the border cell enhancer trap line slbo 1 was examined in stage 10 N ts egg chambers that had been maintained at the restrictive temperature of 32°C. These conditions produce a penetrant oocyte polarization phenotype in which the germinal vesicle often remains at the posterior of the oocyte. Surprisingly, slbo is expressed in neither the anterior nor posterior follicle cells of these egg chambers. In addition, the anterior most follicle cells in N ts mutant egg chambers never round up or migrate between the nurse cells towards the oocyte, indicating that Notch activity is required for border cell development (Gonzalez-Reyes, 1998).

To determine whether Notch activity is required for the specification of other anterior follicle cell types, this experiment using enhancer trap line s to label all anterior follicle cells. N ts egg chambers lack both border and centripetal follicle cells, but these missing cells do not appear to be transformed into stretched follicle cells. Notch therefore seems to play a role in determining the size of the anterior terminal follicle cell population, as well as its subdivision into distinct cell types. The N ts mutant fails to disrupt the patterning of the posterior terminal follicle cells, confirming that Notch is not required for the determination of posterior identity. However, many fewer cells express a posterior marker than in the control egg chambers. This reduction in posterior follicle cell number indicates that Notch plays a role in specifying the size of the terminal follicle cell population that is competent to respond to Gurken and is consistent with the decrease in terminal follicle cell number observed at the anterior of these egg chambers (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).

The neurogenic gene brainiac is specifically required for epithelial development. egghead (egh), a gene with phenotypes identical to brn, encodes for a novel, putative secreted or transmembrane protein. By comparing the function of germline egh and brn to N during oogenesis, direct evidence has been found for the involvement of Notch in maintenance of the follicle cell epithelium, and the specificity of brn and egh in epithelial development during oogenesis. The expression patterns and functional requirements of brn, egh, and N lead to a proposal that these genes mediate follicular morphogenesis by regulating germline-follicle cell adhesion. This proposal offers explanations for (1) the involvement of egh and brn in N-mediated epithelial development, but not lateral specification; (2) why brn and egh embryonic neurogenic phenotypes are not as severe as N phenotypes, and (3) how egh and brn influence Egfr-mediated processes. The correlation between the differential expression of egh in the oocyte and the differential requirement for brn, egh, and N in maintaining the follicular epithelium around the oocyte, suggests that Egghead is a critical component of a differential oocyte-follicle cell adhesive system (Goode, 1996).

Somatically expressed cut interacts with Notch to regulate egg chamber formation and to maintain germline cyst integrity during Drosophila oogenesis. Communication between the germline and the soma during Drosophila oogenesis is essential for the formation of egg chambers and to establish polarity in the developing oocyte. Cut expression initiates in somatic follicle cells in region 2b of the germarium, at about the same time that follicle cells interleave and surround germline cysts. It persists in all follicle cells, including the polar follicle cells and the interfollicular stalk cells, until about stage 6. Between stages 6 and 9 of oogenesis, Cut expression ceases in all the follicle cells except the polar follicle cells. At about stage 10, Cut expression resumes, first in the anterodorsal follicle cells, then throughout the layer of columnar follicle cells which surround the oocyte, and continues in these cells until stage 14 (Jackson, 1997).

Genetic manipulations of cut activity results in defective packaging of germline-derived cysts into egg chambers and disintegration of the structural organization of oocyte-nurse cell complexes to generate multinucleate germline-derived cells. Although these egg chambers invariably

contain 16 germline-derived nuclei, the total number of ring canals is frequently decreased to 14 or 13. The distribution of ring canals is also abnormal in affected egg chambers. cut null alleles in combination with wimp result in egg chamber defects. wimp encodes the RNA polymerase II 140 kD subunit. Ectopic cut expression produces compound egg chamber. cut interacts genetically with the Notch gene and with the catalytic subunit of Protein kinase A gene during egg chamber morphogenesis. cut null mutations suppress loss of Notch function during oogenesis. Two different cut null mutations reduce the incidence of compound egg chambers found in mutant Notch ovaries. Since cut expression is restricted to the somatic follicle cells and cut mutant germline clones are phenotypically normal, it is proposed that the defects in the assembly of egg chambers and the changes in germline cell morphology observed in cut mutant egg chambers are the result of altered interactions between follicle cells and germline cells. It is suggested that cut participates in intercellular communications by regulating the expression of molecules that directly participate in this process (Jackson, 1997).

A new Drosophila gene called toucan, has been characterized that is expressed and required in germline cells to promote proper differentiation of the somatic follicle cells. In wild-type ovaries, Toucan mRNA is first detected in the germarium as soon as the 16-cell cysts are formed. The pattern is maintained in stage 1 and 2 egg chambers. During mid-oogenesis (stage 3 to stage 8), the Toucan mRNA is restricted to the most posterior part of the oocyte. Expression then becomes undetectable in the oocyte but starts in the nurse cells at stage 9. This expression increases strongly during stage 10 and Toucan mRNA again accumulates at the posterior end of the oocyte. All the Toucan mRNA in the nurse cells enters the oocyte at the end of oogenesis, leading to a uniform distribution in early embryos. toucan mutant ovaries are defective in (1) the enclosure of newly formed germline cysts by the follicle cells, (2) the formation of interfollicular stalks, (3) the migration of the follicle cells over the oocyte and (4) the formation of the eggshell. Overexpression of a toucan cDNA in the germline leads to the production of longer interfollicular stalks than wild-type ovaries, a phenotype that is the exact opposite of the toucan mutant phenotype. This observation shows that the formation of the interfollicular stalks depends not only on interactions among the somatic cells but also requires a germline signal. Dominant interactions have been observed between toucan and certain alleles of the daughterless, Notch and Delta genes, each of which is required in the somatic cells for the formation of egg chambers. toucan codes for a large protein with a coiled-coil domain but has no other homology with known proteins. It is proposed that toucan participates in the production or localization of a germline-specific signal(s) that is required for the patterning of the follicular epithelium. It is reasonable to propose that toucan regulates the production and/or distribution of one germline signal necessary to coordinate the rate of somatic cell precursor divisions with the rate of 16-cell cyst production. Based on the results of constitutively active Notch expression, it has been proposed that stalk cell fate depends on a binary decision among the somatic cells (the instructive model). However, in the long stalks produced by germline Toucan overexpression, all of the interfollicular cells express the 93F stalk cell marker. Althogether, these results are consistent with the prohibitive mode in which the somatic cells are maintained in an uncommitted state by the Notch signaling pathway and are induced to differentiate by external signals, i.e. a germline signal involving Toucan (Grammont, 1997).

Two small subgroups of follicle cells have been central to several genetic investigations: the polar cells and the stalk cells. Polar cells are sets of follicle cells located at the anterior and posterior tips of an egg chamber; stalk cells are a linear group of follicle cells that separate the egg chambers. Differentiation of these subgroups occurs when follicle cell intercalation separates egg chambers from the germarium, where germ line division takes place. The fates of these two follicle cell subgroups appear to be linked: mutations in Notch, Delta, fs(1)Yb, or hedgehog cause simultaneous defects in the specification of stalk cells and polar cells. Both of these subgroups are determined in the germarium, and both cease division early in oogenesis. To test the possibility that these subgroups are related by lineage, dominantly marked mitotic clones in ovaries were generated. Small, restricted clones in stalk cells and polar cells are found adjacent to each other at a frequency much too high to be explained by independent induction. A model is therefore proposed in which stalk cells and polar cells are derived from a precursor population that is distinct from the precursors for other follicle cells. This model is supported and extended by characterization of mutants that affect stalk and polar cell formation. Ectopic expression of Hedgehog can induce both polar and stalk cell fate, presumably by acting on the precursor stage. In contrast, stall affects neither the induction of the precursors nor the decision between the stalk cell and polar cell fate but, rather, some later differentiation step of stalk cells. In addition, ectopic polar and stalk cells disturb the anterior-posterior polarity of the underlying oocyte (Tworoger, 1999).

A group of mutants that share a morphological defect in stalk formation have been identified. Loss of function of Notch, Delta, fs(1)Yb, daughterless, hedgehog, toucan, or stall results in large germaria that accumulate cysts and do not form stalks; overexpression of constitutively active Notch (caN), Delta, toucan, or hh results in long, stalk-like structures. Having now shown that stalk cells and polar cells share a distinct precursor, it was asked if the lineage decision between these two groups is affected in each of the mutants bearing a morphological defect in stalk formation. Lack of Notch activity is known to result in an excess of polar cells and loss of stalk cells because of a defect in a precursor stage. Ectopic expression of Hedgehog in the germarium prolongs the precursor stage for stalk cells and polar cells. In oogenesis, hypomorphic alleles of hh produce large germaria that accumulate germline cells. Transient ectopic expression of hh results in long, stalk-like structures and an excess of polar cells in ectopic positions. The cell fates were analyzed in these situations in more detail using a persistent expression system. Receipt of the Hh signal was followed by a ptc enhancer trap line. Two days after Hh induction, long, stalk-like structures expressing the ptc marker were detected. To analyze the identity of the cells in these long, stalk-like structures, they were stained with Big Brain antibody. Bib marks the precursors for stalk cells and polar cells, but not the cells that surround the rest of the egg chamber. The long, stalk-like structures generated by overproduction of Hh do not contain differentiated polar cells or stalk cells. Instead, the follicle cells between 16 cell cysts resemble wild-type precursor cells for these populations in morphology and expression of Bib and FasIII proteins. However, not all of these cells remain in a precursor stage: at later time points, some of the cells in the long stalks 'leak through' the Hh-induced precursor block and differentiate to form extra stalk or polar cells, further evidence that these cells are precursors for polar and stalk cells (Tworoger, 1999).

Hedgehog can induce ectopic stalk cells and polar cells that coincide with a defective oocyte anterior-posterior axis: Three days after persistent Hh induction, patchy patched expression, indicative of Hh action, extends to later-stage egg chambers. Interestingly, in addition to previously observed ectopic polar cells, ectopic stalk cells were observed in these egg chambers, suggesting that ectopic Hh can induce both fates. Therefore, ectopic Hh affects not only the stalk cell and polar cell fates in their normal location, but it can also induce these fates in ectopic locations, probably by transiently inducing the stalk and polar cell precursor stage. Clonal analysis has shown that stalk and polar cells have a common precursor that is separate from a precursor for egg chamber follicle cells. These data show that ectopic Hh has a capacity to induce this precursor fate (Tworoger, 1999).

Coinciding with the ectopic polar cells, an anterior-posterior axis defect is detected in the underlying oocyte. The typical migration of the oocyte nucleus from the posterior to a dorsal-anterior location fails to occur. In addition, Kin:ßgal fusion protein fails to localize posteriorly, and the oocyte microtubule network is defective. These phenotypes have been previously observed in mutants that compromise the Epidermal growth factor receptor or Notch pathways in posterior follicle cells, thereby altering the follicle cell-oocyte signaling that is required for proper anterior-posterior axis formation. stall mutants display defects in egg chamber separation from the germarium that are morphologically comparable to the Notch loss-of-function defect. It was asked whether this defect is caused by the elimination of stalk cell fate, as in Notchts mutants. The expression of the enhancer trap line 93F, which marks stalk cells, was examined in stall mutants. Although no stalk structures are detected, groups of 93F-positive cells are observed in the mutant germaria, suggesting that cells continue to acquire stalk cell fate. The ratio of 93F-positive cells to germline cysts found in stall germaria is consistent with the number of cells found in wild-type stalks. Because 93F marks the terminal filament cells as well as the stalk cells in wild-type ovaries, it is formally possible that the 93F-positive cells in stall germaria represent terminal filament cells. To rule out this possibility, stall mutant ovaries were examined with an independent terminal filament marker. Visualization of stall mutant germaria displays only the wild-type terminal filament expression pattern. In addition, some cells that express an exclusive stalk cell marker were detected in stall mutant germaria. It was concluded that the 93F-positive cells in stall mutant germaria are stalk cells. Furthermore, no obvious defect in the polar cell population is detected in stall mutant germaria. These data suggest that the early lineage decision between polar cell and stalk cell fate occurs normally in the stall mutant. It is concluded that stalk cells are defective in a subsequent differentiation or migration step (Tworoger, 1999).

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).

The body axes of Drosophila are established during oogenesis through reciprocal interactions between the germ line cells and the somatic follicle cells that surround them. The Notch pathway is required at two stages in this process: first, for the migration of the follicle cells around the germ line cyst and, later, for the polarization of the anterior-posterior (A-P) axis of the oocyte. Its function in these events, however, has remained controversial. Using clonal analysis, it has been shown that Notch signaling controls cell proliferation and differentiation in the whole follicular epithelium. Notch mutant follicle cells remain in a precursor state and fail to switch from the mitotic cell cycle to the endocycle. Furthermore, removal of Delta from the germ line produces an identical phenotype, showing that Delta signals from the germ cells to control the timing of follicle cell differentiation. This explains the axis formation defects in Notch mutants, which arise because undifferentiated posterior follicle cells cannot signal to polarize the oocyte. Delta also signals from the germ line to Notch in the soma earlier in oogenesis to control the differentiation of the polar and stalk follicle cells. The germ line therefore regulates the development of the follicle cells through two complementary signaling pathways: Gurken signals twice to control spatial patterning, whereas Delta signals twice to exert temporal control (Lopez-Schier, 2001).

The observation that loss of Notch activity does not lead to the formation of extra polar cells raises the question of whether Notch is actually required in the polar follicle cells themselves. A screen was carried out for Notch mutant clones in these cells using the polar cell-specific marker A101. In control experiments, wild-type clones were recovered that include the polar follicle cells in 18% of the egg chambers, whereas no case were found in which a Notch mutant cell expressed A101 out of 200 egg chambers screened. In addition, clones were recovered in the stalk cells. Thus, the Notch pathway seems to be required for the differentiation of the polar and stalk follicle cells, as well as the epithelial cells (Lopez-Schier, 2001).

Several lines of evidence indicate that the encapsulation of the germ line cysts depends on the polar and stalk cells. Consistent with this, adjacent egg chambers are often fused in ovaries containing Notch mutant clones, and the polar cells and stalk cells are always absent in these cases. In contrast, large Notch mutant clones in the epithelial follicle cell layer have no effect on cyst encapsulation. In rare cases, adjacent egg chambers are only partially fused, with a single or double layer of follicle cells between them. In these examples, the boundary between the partially fused cysts is covered by mutant epithelial follicle cells, but there are no A101 positive cells where the polar follicle cells would be expected to lie. In contrast, adjacent cysts are separated normally when the polar cells are wild type but the epithelial cells are mutant. These results are consistent with a model in which Notch pathway mutants disrupt the encapsulation of the germ line cysts because Notch signaling is required for the differentiation of the polar and stalks cells that mediate this process (Lopez-Schier, 2001).

The differentiation of the epithelial follicle cells is first apparent at stage 7 and occurs after the cells have exited the mitotic cell cycle and entered the endocycle to become polyploid. Because Notch mutations arrest these cells at the precursor stage, whether they also prevent this switch in the cell cycle was examined by analyzing the division patterns of mutant clones. At later stages of oogenesis, mutant clones always contain many more cells than the wild-type twin spot clones that were induced at the same time. The mutant clones still occupy the same area as the twin spot clones, however, because the Notch mutant cells are much smaller than their wild-type siblings. Thus, the loss of Notch causes the cells to go through extra cell divisions, without a corresponding increase in growth rate, to generate a larger number of smaller cells (Lopez-Schier, 2001).

To determine when these extra divisions occur, an antibody against the phosphorylated form of the Histone-H3, which labels cells in late G2 and mitosis but not cells in the endocycle, was used. Before stage 6, there is no obvious difference between the frequency of mitoses in wild-type and mutant cells. Wild-type cells never stain for phospho-Histone H3 after stage 6, however, whereas mutant cells continue to divide up until stages 10B or 11. Furthermore, mutant cells have a lower DNA content and smaller nuclei than do wild-type cells. These results indicate that Notch mutant cells fail to switch from the mitotic cell cycle to the endocycle, and carry on dividing instead of becoming polyploid (Lopez-Schier, 2001).

Clonal analysis of Delta mutation indicates that Delta signals from the germ line to activate Notch in the somatic follicle cells twice during oogenesis: once in the germarium to induce the differentiation of the polar/stalk cell lineage and then later to induce the differentiation of the epithelial follicle cells. To see if this correlates with expression of the two proteins, wild-type ovaries were stained for Notch and Delta. Notch protein is expressed from the germarium up to stage 7 and localizes to the apical membrane of the follicle cells, in close contact with the germ line. This apical localization disappears after this stage, leaving behind a faint punctate cytoplasmic staining. Delta protein expression shows a different but complementary pattern of expression. Delta is expressed by the germ line and the soma, but it is particularly abundant in the nurse cells and oocyte, where it seems to accumulate at the plasma membrane, and in large particles in the cytoplasm. Delta protein is present at low levels during early oogenesis but increases in abundance from stage 5 onward to reach its highest levels at stage 7, right at the time when Notch protein disappears from the apical membrane of follicle cells. This suggests that there is a burst of Delta signaling during stages 5 through 7, which removes most Notch protein from the membrane, either because the receptor is cleaved on binding to Delta or because Notch is down-regulated in response to the activation of the pathway. In either case, the loss of Notch protein from the follicle cell membranes should depend on Delta signaling, and the distribution of Notch protein was therefore examined in Delta germ line clones, using antibodies directed against the extracellular and intracellular portions of the receptor. Both antibodies reveal that Notch is not down-regulated in Delta mutant egg chambers. This is particularly clear in chimaeric egg chambers that contain both wild-type and mutant germ cells, where Notch remains associated with the apical membrane of all follicle cells in contact with the mutant germ cells but disappears from those overlying the wild-type germ line cells. This suggests that the Delta signals to the epithelial follicle cells at around stages 5 to 7, which coincides with when these cells cease dividing and start to differentiate (Lopez-Schier, 2001).

Drosophila oogenesis provides an excellent system in which to analyze the Notch signaling pathway for several reasons. (1) Delta signals to activate Notch in a large number of follicle cells at the same time, because ~1000 epithelial follicle cells receive the second signal during stages 5 through 7. (2) The epithelial cells constitute one of the rare examples where the down-regulation of Notch in response to Delta can be observed directly, and this allows one to see when and where signaling takes place. (3) The cells that send the signal are clearly distinct from the cells that receive it, because Notch is not required in the germ line, nor Delta in the epithelial cells. The germ cells form a separate lineage from the rest of the organism very early in embryogenesis, and it is therefore straightforward to determine whether other genes in the pathway act in the signaling or responding cells. Two other neurogenic genes, egghead and brainiac, have previously been shown to produce Notch-like phenotypes during oogenesis. Because both genes are required in the germ line, this leads to the clear prediction that they are involved in the production of functional Delta. Brainiac shows sequence similarity to Fringe, and related mammalian proteins have been characterized as glucosyltransferases. This raises the interesting possibility that Brainiac is a glucosyltransferase that adds sugar residues to Delta, in much the same way that Fringe modifies Notch (Lopez-Schier, 2001).

fringe encodes a glycosyltransferase that modulates the ability of the Notch receptor to be activated by its ligands. fringe functions during early stages of Drosophila oogenesis. Animals mutant for hypomorphic alleles of fringe contain follicles with an incorrect number of germline cells, which are separated by abnormally long and disorganized stalks. Analysis of clones of somatic cells mutant for a null allele of fringe localizes the requirement for fringe in follicle formation to the polar cells, and demonstrates that fringe is required for polar cell fate. Clones of cells mutant for Notch also lack polar cells and the requirement for Notch in follicle formation appears to map to the polar cells. Ectopic expression of fringe or of an activated form of Notch can generate an extra polar cell. These results indicate that fringe plays a key role in positioning Notch activation during early oogenesis, and establish a function for the polar cells in separating germline cysts into individual follicles (Grammont, 2001).

The phenotypes observed in fng mutant clones are also observed in Notch mutant clones. Moreover, as for fng, the requirements for Notch appear to map to the polar cells. While these results confirm that Notch plays important roles in separating follicles and organizing stalks, they overturn previous conclusions about the nature of this requirement. Most notably, it had been suggested that loss of Notch function leads to extra polar cells. Although after stage 8 Fas III becomes expressed in Notch mutant cells, this occurs regardless of their location and in the absence of other aspects of polar cell fate. Instead, the results described here imply that Notch is required for polar cell specification, and that Notch is required only in these cells for the separation of germline cysts into distinct follicles. Furthermore, the specific expression of E(spl)mß-lacZ in the polar cells confirms that Notch is activated within these cells, and the induction of extra polar cells by activated Notch demonstrates that the activation of Notch can be sufficient to specify polar cells within a competent subpopulation of somatic cells (Grammont, 2001).

In the Drosophila wing and eye, as well as in cultured mammalian cells, fng has been shown to potentiate the activation of Notch by Delta and to inhibit the activation of Notch by Serrate. Notch, Serrate and Delta all appear to be expressed ubiquitously in follicle cells throughout the germarium. In many circumstances, expression of Notch ligands can exert an autonomous inhibition of Notch activation. It has been hypothesized that fng may potentiate Delta signaling by allowing Notch activation within ligand-expressing cells. Similarly, it is proposed that in the germarium, fng expression overrides autonomous inhibition to allow Notch activation by Delta within ligand-expressing cells, thereby positioning Notch activation to a subset of cells (the polar cells) within a broad domain of Notch and ligand expression. This is consistent with the observation that Notch and fng are both positively required within polar cells for them to adopt their fate, and that fng expression soon becomes restricted to these cells (Grammont, 2001).

Throughout Drosophila oogenesis, specialized somatic follicle cells perform crucial functions in egg chamber formation and in signaling between somatic and germline cells. In the ovary, at least three types of somatic follicle cells, polar cells, stalk cells and main body epithelial follicle cells, can be distinguished when egg chambers bud from the germarium. Although specification of these three somatic cell types is important for normal oogenesis and subsequent embryogenesis, the molecular basis for establishment of their cell fates is not completely understood. Studies reveal the gene eyes absent (eya) to be a key repressor of polar cell fate. Eya is a nuclear protein that is normally excluded from polar and stalk cells, and the absence of Eya is sufficient to cause epithelial follicle cells to develop as polar cells. Furthermore, ectopic expression of Eya is capable of suppressing normal polar cell fate and compromising the normal functions of polar cells, such as promotion of border cell migration. Finally, it has been shown that ectopic Hedgehog signaling, which is known to cause ectopic polar cell formation, does so by repressing eya expression in epithelial follicle cells (Bai, 2002).

One germline signal that is known to play a role in polar cell specification is Delta, which signals from the germline to Notch in the soma to control the differentiation of polar cells. Epithelial follicle cells do not respond to Delta in the same way, presumably because, unlike polar cells, they do not express fringe. fringe encodes a glucosyltransferase that potentiates the ability of the Notch receptor to be activated by its ligand, Delta. Mutation of either Notch or fringe leads to the disappearance of polar cells. As a result, Eya-negative cells are not found in the follicles. Mis-expression of either Fng or activated Notch produces ectopic polar cells only at the poles of the egg chamber, whereas loss of Eya can cause polar cells to form throughout the follicle epithelium. Thus Notch signaling appears to be necessary, but not sufficient to repress Eya expression and leads to polar cell formation. Surprisingly, activated Notch also can produce ectopic polar cells cell-nonautonomously at the poles of the egg chamber. The reason for this could be that activated Notch signaling might activate the expression of Delta, which, in turn, can activate Notch signaling in the adjacent cells (Bai, 2002).

The anterior-posterior axis of Drosophila becomes polarized early in oogenesis, when the oocyte moves to the posterior of the germline cyst because it preferentially adheres to posterior follicle cells. The source of this asymmetry is unclear, however, since anterior and posterior follicle cells are equivalent until midoogenesis, when Gurken signaling from the oocyte induces posterior fate. Asymmetry is shown to arise because each cyst polarizes the next cyst through a series of posterior to anterior inductions. Delta signaling from the older cyst induces the anterior polar follicle cells, the anterior polar cells signal through the JAK/STAT pathway to induce the formation of the stalk between adjacent cysts, and the stalk polarizes the younger anterior cyst by inducing the shape change and preferential adhesion that positions the oocyte at the posterior. The anterior-posterior axis is therefore established by a relay mechanism, which propagates polarity from one cyst to the next (Torres, 2003).

The ovary of Drosophila is composed of about 16-20 ovarioles, each of which contains a series of egg chambers that proceed through the 14 stages of oogenesis as they move from the anterior germarium toward the oviduct at the posterior. The germline stem cells reside at the anterior tip of the germarium and divide asymmetrically to produce a new stem cell and a cystoblast, which then undergoes four consecutive mitoses with incomplete cytokinesis to give rise to a cyst of 16 interconnected germ cells. One of these cells is selected to become the oocyte and moves to the posterior of the cyst in region 3 of the germarium. This asymmetric arrangement of the germ cells generates the first anterior-posterior (A-P) polarity in development and leads to the polarization of the A-P axis of the embryo through two signaling events between the oocyte and the somatic follicle cells. At stage 6, Gurken signals from the oocyte to induce the adjacent follicle cells to adopt a posterior rather than an anterior fate. The posterior cells then send an unknown signal back to the oocyte at stage 7 to induce the formation of a polarized microtubule cytoskeleton, which directs the transport of bicoid mRNA to the anterior of the oocyte and of oskar mRNA to the posterior. The localization of these transcripts defines the A-P axis of the embryo, since bicoid mRNA encodes the anterior morphogen that patterns the head and thorax of the embryo, and oskar mRNA defines the site of formation of the pole plasm, which contains the abdominal and germline determinants (Torres, 2003 and references therein).

Mutants that disrupt the movement of the oocyte to the posterior of the cyst give rise to bipolar egg chambers with symmetric oocytes that localize bicoid mRNA to both poles and oskar mRNA to the center, indicating that all subsequent anterior-posterior asymmetries depend on the positioning of the oocyte. This morphogenetic movement occurs as the cyst moves from region 2b to 3 of the germarium. The cyst flattens to form a lens-shaped disc in region 2b, and somatic follicle cells migrate to separate the cyst from the preceding older egg chamber. As the cyst enters region 3, it rounds up with the oocyte at the posterior and eventually protruding into the surrounding follicle cell layer. This process requires the preferential adhesion of the oocyte to the follicle cells that surround the posterior of the cyst. Both the oocyte and these follicle cells independently upregulate the homophilic adhesion molecule Shotgun, and removal of Shotgun from either cell disrupts oocyte positioning. This has led to a model in which the upregulation of Shotgun in the oocyte allows it to outcompete the nurse cells for adhesion to the posterior follicle cells, thereby anchoring it at the posterior, as the cyst changes shape. Thus, the first cue for anterior-posterior polarity is the increased adhesiveness of the posterior follicle cells, although it is not known why these cells behave differently from the other follicle cells (Torres, 2003 and references therein).

The follicle stem cells reside in region 2b of the germarium and give rise to two distinct lineages: the epithelial follicle cell precursors, which proliferate until stage 6 to generate most of the cells that surround each cyst, and the polar/stalk precursors. The latter exit mitosis at stage 1 to 2 of oogenesis and give rise to the symmetric pairs of polar cells at the anterior and posterior poles of the cyst and to the stalk that separates each cyst from the adjacent one. Delta mutant germline clones and Notch follicle cell clones fail to form polar cells, indicating that Delta signals from the germline to activate the Notch receptor in the polar/stalk precursors to induce them to adopt the polar cell fate. This induction requires fringe, which is upregulated in the polar/stalk precursors and renders these precursors competent to respond to the Delta signal. Once the polar cells are specified, they express Unpaired, the ligand for the JAK/STAT pathway, and the resultant activation of JAK/STAT signaling plays two key roles in patterning the rest of the follicle cells. (1) The polar cells induce uncommitted polar/stalk cell precursors to become stalk cells. Overexpression of Unpaired causes all polar/stalk cell precursors to differentiate as stalk, whereas loss-of-function mutations in hopscotch (JAK) or STAT92E cause a loss of the stalk. (2) Unpaired signaling from the polar cells induces the adjacent epithelial follicle cells at each pole of the egg chamber to adopt a terminal fate. This induction is essential for axis formation because only the terminal cells are competent to respond to Gurken by becoming posterior. Unpaired also acts as a morphogen to specify three distinct terminal cell types at the anterior: the border cells, the stretched follicle cells, and the centripetal cells. In the absence of Gurken signaling, all three cell types also form at the posterior of the egg chamber, indicating that the graded activity of JAK/STAT pathway creates a symmetric prepattern at both poles (Torres, 2003 and references therein).

The analysis of follicle cell patterning raises an intriguing paradox. On the one hand, the anterior-posterior polarity of the follicle cell layer depends on the positioning of the oocyte, since this determines the direction of the Gurken signaling that makes the posterior cells different from the anterior ones. On the other hand, the positioning of the oocyte depends on the fact that oocyte adheres more strongly to the posterior follicle cells than to the other follicle cells, indicating that these posterior cells must already be different before the oocyte is positioned. Although the follicle cells that adhere to the oocyte have not been unambiguously identified, it has recently been proposed that they correspond to the posterior polar cells. In order to investigate the source of this early asymmetry, the function of the polar cells has been analyzed by disrupting Delta signaling from single germline cysts. The results resolve this paradox by showing that the anterior and posterior polar cells are not equivalent in the germarium. However, the positioning of the oocyte does not depend on the posterior polar cells but on the anterior polar cells of the adjacent older cyst and the stalk that they induce. This leads to the proposal of a novel model for anterior-posterior axis formation in Drosophila, in which each cyst transmits polarity to the adjacent younger cyst (Torres, 2003).

It has been thought that the polar and terminal follicle cells at each end of the egg chamber are equivalent until around stage 5 to 6 of oogenesis, when Gurken signals from the oocyte to induce the adjacent cells to adopt a posterior fate. The current results reveal, however, that there are a number of differences between the anterior and posterior pairs of polar cells when they differentiate during stages 1 to 2: (1) these cells arise asynchronously, since the anterior polar cells appear about 12 hr before the posterior ones; (2) about four cells initially express polar cell markers at the anterior, whereas only two, or very occasionally three, cells express these markers at the posterior; (3) only the anterior polar cells are competent to induce the formation of the stalk, since normal stalks form at the posterior of a Delta mutant cysts, which lack posterior polar cells, and (4) posterior polar cells are not sufficient for stalk formation, since they are unable to induce a stalk when the older cyst lacks anterior polar cells (Torres, 2003).

It is unclear why posterior polar cells differentiate later than the anterior ones, since both depend on the same Delta signal from the germline. One possibility is that this difference occurs because the Delta signal itself is asymmetric. The oocyte has already reached the posterior by the time that this induction occurs, and it is therefore possible that Delta signals more strongly or earlier from the nurse cells at the anterior of the germline cyst than from the posterior oocyte. Alternatively, the asynchrony could reflect an intrinsic difference in the competence of the polar cell precursors to respond to Delta. Clonal analysis indicates that the precursors of the anterior polar cells of one cyst, the stalk cells, and the posterior polar cells of the adjacent younger cyst are closely related, indicating that they all arise from the group of polar/stalk precursors that migrate between the cysts in region 2b of the germarium. Thus, the posterior polar cell precursors migrate at the same time as those of the anterior polar cells of the adjacent older cyst, whereas the precursors of the anterior polar cells of the same cyst only arrive about 12 hr later, when the next cyst reaches region 2b. This means that posterior polar cell precursors are significantly older than their anterior counterparts, and this may reduce their competence to respond to the Delta signal, leading to a delay in their differentiation. One reason why this might occur is because these cells have already been exposed to Unpaired signaling from the anterior polar cells of the older cyst and have therefore started to differentiate as stalk, which may make it more difficult to induce them to switch into the polar cell pathway (Torres, 2003).

The difference in the timing of the differentiation of anterior and posterior polar cells probably accounts for all the other asymmetries between these two sets of otherwise identical cells. For example, the posterior polar cells may be unable to induce the stalk because, by the time they are specified, the polar/stalk cell precursors have lost their competence to respond to the inductive signal. Activation of the JAK/STAT pathway is both necessary and sufficient to induce polar/stalk precursors to adopt the stalk cell fate, and the ligand for this pathway, Unpaired, is expressed in both sets of polar cells. The posterior polar cells express Unpaired only at stage 2, however, whereas the stalk is induced about a day earlier, when the cyst enters region 3 of the germarium. Although the expression of Unpaired by the posterior polar cells plays no role in stalk formation, it is not redundant because Unpaired also induces the epithelial follicle cells around the polar cells to adopt a terminal fate, and this is essential later in oogenesis to render these cells competent to respond to Gurken by becoming posterior. The delay in the differentiation of the posterior polar cells can also explain why only two, or very occasionally three, cells are initially specified at this end of the egg chamber, compared to the four or more cells that arise at the anterior, since most of the stalk/polar precursors have already adopted the stalk fate by this stage, and there will therefore be many fewer uncommitted precursors that are still competent to become polar cells. Finally, this delay can account for the later differentiation of the posterior polar cells, which is reflected in the fact that they round up and detach from the stalk during stages 4 to 5, whereas the anterior polar cells do this during stages 2 to 3. Thus, all of the differences between the anterior and posterior polar cells are temporal, and the two pairs of cells are identical in terms of their differentiation and gene expression from stage 2 until stage 6, when Gurken signals to break this symmetry (Torres, 2003).

The positioning of the oocyte at the posterior of the germline cyst generates the first anterior-posterior polarity in Drosophila development and ultimately leads to the formation of the A-P axis of the embryo. The results indicate that the asymmetric induction of the polar cells plays a key role in generating this polarity and leads to the proposal of a model in which each cyst induces the positioning of the oocyte in the following younger cyst. (1) By the time a germline cyst reaches region 3 of the germarium, it has already positioned its oocyte to the posterior and is separated from the adjacent younger cyst in region 2b by a pool of uncommitted polar/stalk precursors. At this point, Delta signals from the germline cyst to activate Notch in the adjacent anterior polar/stalk precursors, thereby inducing them to develop as polar cells. (2) These anterior polar cells turn on Unpaired and induce the more anterior polar/stalk precursors to differentiate as stalk cells. (3) The stalk cells intercalate with each other and converge toward the middle of the ovariole to generate a two cell-wide stalk. This morphogenetic movement causes the younger anterior cyst to round up, as it is pulled into region 3. In parallel, the stalk somehow induces the upregulation of Shotgun in the follicle cells that contact the oocyte of the younger cyst, and they therefore adhere preferentially to this cell. This positions the oocyte by causing it to protrude into the follicle cell layer, which anchors it at the posterior as the cyst changes shape. (4) By the time that the younger cyst has positioned its oocyte, it has entered region 3 of the germarium and activated Delta signaling. This then induces polar cell fate in the polar/stalk precursors that have migrated to cover its anterior, and the cycle begins again. It is only at this stage that polar cells differentiate at the posterior of the older cyst, which has now exited the germarium and is at stage 2 (Torres, 2003).

The oocyte is therefore positioned by a relay mechanism that involves a series of posterior to anterior inductions. The older cyst induces the anterior polar cells, the anterior polar cells induce the stalk, and the stalk induces the positioning of the oocyte of the younger anterior cyst. Each ovariole functions as a production line for new egg chambers, since the germline stem cells lie at one end and divide to provide a constant source of new cysts. This relay can therefore be repeated over and over again to position the oocyte at the posterior of each new cyst that passes through the germarium. In this way, anterior-posterior polarity is transmitted from one cyst to the next, to define the anterior-posterior axis of each egg chamber. This type of relay represents a simple mechanism for propagating a repeated asymmetric pattern (Torres, 2003).

One open question that remains is the identity of the cells that preferentially adhere to the oocyte to anchor it at the posterior. Although it has been previously proposed that the posterior polar cells fulfill this function, several pieces of evidence demonstrate that they are not required for oocyte positioning. (1) The oocyte is always correctly localized at the posterior of Delta mutant cysts, which lack posterior polar cells. Large Notch mutant follicle cell clones at the posterior of the egg chamber also block posterior polar cell specification but have no effect on oocyte positioning. (2) The oocyte always becomes mislocalized in the wild-type cysts anterior to a Delta mutant cyst, even though the posterior polar cells are present, indicating that these cells are not sufficient for oocyte positioning. In fact, the oocyte shows no preference for contact with the posterior polar cells, when it moves to the side of the cyst. (3) The oocyte is positioned as it enters region 3 of the germarium, while the posterior polar cells are only specified a day later at stage 2. (4) Electron micrographs reveal that the oocyte contacts about six follicle cells as it protrudes from the posterior of the cyst on entering region 3, whereas there are only two polar cells (Torres, 2003).

Since the posterior polar cells do not position the oocyte, some other cells must fulfill this function. One possibility is that the stalk induces the epithelial follicle cells at the posterior of the cyst to upregulate Shotgun and adhere to the oocyte. Alternatively, the stalk cells could adhere to the oocyte directly. This would be consistent with the fact that all stalk cells upregulate Shotgun as the stalk forms and remove the requirement for a third induction from the stalk to the epithelia cells. Furthermore, such a direct contact would help to explain why the formation of the stalk causes the anterior cyst to round up. The cells that eventually become the posterior polar cells may also participate in this adhesion, and then switch to the polar fate at stage 2, because they remain in contact with the oocyte and are therefore exposed to the next round of Delta signaling (Torres, 2003).

The relay between cysts represents the most upstream event in Drosophila axis formation that has been discovered so far and can account for the origin of anterior-posterior polarity in the vast majority of egg chambers. It cannot explain, however, how the first cyst in each ovariole is polarized. This leads to the prediction that other cells must fulfill the function of the posterior stalk in positioning the oocyte of this first cyst, and good candidates would be the basal stalk cells of the pupal ovariole. It is worth noting that the polarization of each cyst does not require that the previous cyst has correctly positioned its oocyte, since wild-type cysts anterior to a Delta mutant cyst induce oocyte positioning in the next cyst, even though their own oocyte is misplaced. Thus, the polarization of the first egg chamber in each ovariole is not necessary for the polarity of all subsequent egg chambers (Torres, 2003).

A role for extra macrochaetae downstream of Notch in follicle cell differentiation

The Drosophila ovary provides a model system for studying the mechanisms that regulate the differentiation of somatic stem cells into specific cell types. Ovarian somatic stem cells produce follicle cells, which undergo a binary choice during early differentiation. They can become either epithelial cells that surround the germline to form an egg chamber ('main body cells') or a specialized cell lineage found at the poles of egg chambers. This lineage goes on to make two cell types: polar cells and stalk cells. To better understand how this choice is made, a screen was carried out for genes that affect follicle cell fate specification or differentiation. extra macrochaetae (emc), which encodes a helix-loop-helix protein, was identified as a downstream effector of Notch signaling in the ovary. Emc is expressed in proliferating cells in the germarium, as well as in the main body follicle cells. Emc expression in the main body cells is Notch dependent, and emc mutant cells located on the main body fail to differentiate. Emc expression is reduced in the precursors of the polar and stalk cells, and overexpression of Emc caused dramatic egg chamber fusions, indicating that Emc is a negative regulator of polar and/or stalk cells. Emc and Notch are both required in the main body cells for expression of Eyes Absent (Eya), a negative regulator of polar and stalk cell fate. It is proposed that Emc functions downstream of Notch and upstream of Eya to regulate main body cell fate specification and differentiation (Adam, 2004).

EMC has a complex pattern of expression. emcP5C-lacZ is expressed in the terminal filament and inner sheath cells of the germarium, and in the main body follicle cells of egg chambers, but not in the early follicle cells or in polar or stalk cells. An anti-Emc antibody revealed additional features of Emc expression: Emc is expressed in the undifferentiated follicle cells of the germarium. Its expression is reduced in follicle cells in the intercyst regions between region 2 and region 3 cysts. It is maintained in main body cells but further reduced in stalk and polar cells of stage-1-4 egg chambers. Its expression in stalk cells remains low, but, in polar cells after stage 4, returns to a level indistinguishable from that exhibited by their neighbors. The overall level of expression is somewhat reduced from stage 6 to stage 8, and at stage 9 expression in the oocyte-associated follicle cells is further reduced, although still detectable (Adam, 2004).

There are number of similarities in the phenotypic effects of Notch and Emc in egg chamber development; it is concluded that Emc is a downstream effector of Notch in the differentiation of follicle cells. In other tissues, genes of the Enhancer of split complex are major downstream effectors of Notch signaling, but they do not appear to be required for differentiation of the follicle cells. By contrast, Emc is expressed in most somatic cells of the ovariole and has a loss-of-function phenotype similar to that of Notch. Like Notch, Emc is required for the formation of polar cells, and egg chambers lacking polar cells are observed at one end when emc clones are generated. emc mutant cells also fail to downregulate Fas3 expression at stage 6, and the cells fail to enter endoreplication at the proper time, indicating that emc is required for follicle cell differentiation. In addition, Emc expression in the main body follicle cells is dependent on Notch and can be induced by forced expression of activated Notch. Taken together, this leads to the proposal that Emc is an effector of Notch activity in follicle cells (Adam, 2004).

Notch and Emc are both involved in separation of adjacent egg chambers. Follicles mutant for Notch or emc include egg chambers containing multiple germline cysts and lacking intervening polar and stalk cells. Recent work has shown that Notch signaling is not required for early packaging (enveloping of the cyst by follicle cells), but its role in specifying polar/stalk precursors, which might occur subsequent to packaging, has not been addressed specifically. However, Notch is required autonomously for polar cell differentiation, and at least one pair of differentiated polar cells is required to induce a stalk. Therefore, the packaging defects observed in Notch mutants may be due solely to either the failure of polar cells to differentiate or to a particular role in specifying polar/stalk precursors. The fused egg chambers observed in emc mutants could also be due to a role in one or both processes. If emc were involved separately in both processes, then a dramatic increase would be expected in the frequency of fused egg chambers when clones were generated in the germarium, where precursors are formed. Since no such increase was seen, a role for emc in polar/stalk precursor formation is not posited; however, without a better understanding of polar/stalk precursors, this cannot be confirmed (Adam, 2004).

emc is probably not the only effector downstream of Notch in the ovariole, since emc mutant clones exhibit some differences from Notch mutant clones. In general the effects of emc are less penetrant than those of Notch. For example, emc mutant cells, like Notch mutant cells, exhibit persistent Phosphohistone H3 and Cyclin B labeling but at low frequency and at low levels. In addition, although emc is involved in polar cell specification or differentiation, it is only partially required for this, since normal polar cells could be observed even in clones of a null emc allele. Thus, there are likely to be additional downstream effectors of Notch in the ovary (Adam, 2004).

Molecular epistasis indicates that Notch signals through Emc to induce or maintain Eya expression in the main body follicle cells. Eya expression is lost in large Notch mutant follicle cell clones and can be induced by forced expression of activated Notch in the follicle cells. Eya is involved in inhibiting polar and stalk cell fate, so one might expect that Notch or emc loss-of-function mutants would make extra polar cells. This is not the case, however; it seems likely that the reason Notch and emc mutant cells do not become polar cells is that they fail to differentiate. Eya does not appear to be required for differentiation, but rather for main body cell fate, since eya mutant cells differentiate into polar cells. Thus, the Notch pathway must branch downstream of Emc, with one pathway leading to Eya expression and repression of polar cell fate, and a separate pathway leading to differentiation (Adam, 2004).

Emc can have both positive and negative effects on the number of polar cells. Although this might initially seem mysterious, it can be explained by the dynamic expression of Emc in ovaries. Emc is expressed in the germarium, but it is reduced in polar/stalk precursors. Its expression remains low in stalk cells but, in polar cells, returns to the same level as that of their neighbors around the time of differentiation. Temperature-shift experiments show that forced expression of Emc in immature polar cells can lead to expression of Eya, which is the presumed cause of loss of polar cells. By contrast, forced expression of Emc in maturing polar cells appears to lead to potentiation of polar cell number, i.e., polar cell number per group is not reduced from four to two, as in wild-type, but remains at four polar cells per group. This is probably due to a role for Emc in polar cell differentiation, because loss-of-function emc clones can result in loss of polar cells (Adam, 2004).

Three lines of evidence suggest that Emc may be a key regulator of Eya expression. (1) Emc and Eya expression are similar in multiple respects. Both are upregulated in follicle cells in region 2B of the germarium, and in main body follicle cells from stages 2 through 6. Both are downregulated in the polar/stalk lineage from the germarium through stage 3, and in the oocyte-associated follicle cells at stages 7 through 9. (2) Emc is required for Eya expression in the main body. (3) Emc and Eya produce similar overexpression phenotypes, including fused egg chambers and the loss of polar cells, and, when Emc is overexpressed in the polar cells, Eya expression is induced. Taken together, these data suggest that the expression of Eya in the follicle cells is largely regulated by Emc. Emc is a helix-loop-helix protein that lacks the basic DNA-binding domain of the bHLH transcription factors. It normally opposes the activity of bHLH transcription factors by sequestering them in non-productive complexes. Thus, the dependence of Eya on Emc is likely to be indirect. Presumably, Emc inhibits a bHLH protein that inhibits expression of Eya. The identity of this protein remains an interesting subject for further study (Adam, 2004).

Hindsight mediates the role of Notch in suppressing Hedgehog signaling and cell proliferation

Temporal and spatial regulation of proliferation and differentiation by signaling pathways is essential for animal development. Drosophila follicular epithelial cells provide an excellent model system for the study of temporal regulation of cell proliferation. In follicle cells, the Notch pathway stops proliferation and promotes a switch from the mitotic cycle to the endocycle (M/E switch). This study shows that zinc-finger transcription factor Hindsight mediates the role of Notch in regulating cell differentiation and the switch of cell-cycle programs. Hindsight is required and sufficient to stop proliferation and induce the transition to the endocycle. To do so, it represses string, Cut, and Hedgehog signaling, which promote proliferation during early oogenesis. Hindsight, along with another zinc-finger protein, Tramtrack, downregulates Hedgehog signaling through transcriptional repression of cubitus interruptus. These studies suggest that Hindsight bridges the two antagonistic pathways, Notch and Hedgehog, in the temporal regulation of follicle-cell proliferation and differentiation (Sun, 2007).

How developmental signals coordinate to control cell proliferation and differentiation remains largely unknown. These data reveal a molecular mechanism that links signal-transduction pathways and the cell-cycle machinery. Hnt is induced by Notch signaling and mediates most, if not all, Notch functions in the downregulation of Hh signaling and the M/E switch in follicle cells during midoogenesis. Loss of hnt function in follicle cells results in an extra round of the mitotic cycle after stage 6 and a delayed entry into the endocycle. In contrast, misexpression of Hnt at an earlier stage causes the follicle cells to differentiate prematurely and enter the endocycle. Hnt suppresses both stg and Cut, whose expression must be downregulated to ensure the M/E switch. In addition, Notch signaling appears to act through Hnt to downregulate Hh signaling by suppressing ci transcription, so Hnt links the two antagonistic signaling pathways in follicle-cell development. The transcriptional repression of ci is probably not mediated by Hnt alone, because ttk exhibited a similar defect in transcriptional regulation of ci and stg (Sun, 2007).

Studies have shown that downregulation of Cut mediates part of Notch function during the M/E switch. Specifically, Cut promotes cell proliferation and maintains an immature-cell fate, but Stg, the Cdc25 homolog, is not regulated by Cut. To induce the mitotic division ectopically during midoogenesis in follicle cells, both Cut and Stg must be misexpressed. The current study suggests that both Cut and Stg are suppressed by Hnt. Without Stg activity, a major regulator of G2/M transition, follicle cells are arrested before they enter the M phase, and downregulation of Cut allows accumulation of Fzr, causing degradation of CycA and CycB by the UPS, thus lowering CDK activity. This process allows endocycling follicle cells to by-pass the M phase and enter the next S phase. Repeated gap phases and S phases constitute the endocycle (Sun, 2007).

The finding that hnt follicle cells enter the endocycle after one additional round of the mitotic cycle suggests that hnt mutation causes a delay in the M/E switch. Mutations of the Notch pathway may also result in only a delay in entering the endocycle. In Notch mosaics, the cell number in mutant clones is approximately twice that of the twin spots, suggesting that an additional cell cycle also takes place. Further testing of this hypothesis requires a detailed analysis of the DNA content and clone size in Notch pathway mutants. Alternatively, Hnt may not be the sole mediator of the Notch effect; for example, Su(H)-independent Notch signaling may also be required in the M/E switch. Although hnt mutant cells can enter the endocycle late, they could not enter the chorion-gene-amplification program even much later, suggesting that Hnt function is also required for chorion-gene amplification (Sun, 2007).

The removal of negative components of the Hh pathway such as ptc causes overproliferation in follicle cells. Loss-of-function analyses of fu, a positive regulator of the pathway, revealed fewer cells in the mutant clones than in twin spots. The nuclear sizes of fu mutant cells were similar to those of the wild-type at the same developmental stage, and no fragmentation of the chromosomes was observed. Hh signaling therefore promotes cell proliferation in follicle cells during early oogenesis. Thus, Hnt-mediated downregulation of Hh signaling through suppression of ci transcription plays an important role in the M/E switch. Hh signaling is probably not involved in regulating Cut or Stg expression, because ectopic expression of Ci-155 in follicle cells during midoogenesis did not extend Stg-lacZ or Cut expression beyond stage 6, and fu mutant follicle cells showed normal Cut expression during early oogenesis. Other factors may therefore mediate the role of Hh signaling to modulate proliferation of follicle cells (Sun, 2007).

Hnt is not only required to mediate the role of Notch in regulating the M/E switch in follicle cells, but it is also sufficient to drive premature entry into the endocycle. Only a few cells misexpressing Hnt at the early stages of oogenesis were recovered, consistent with the role of Hnt in terminating the mitotic phase. In an extreme case, a stage-4 egg chamber contained only ~20 follicle cells, most of which misexpressed Hnt. Hnt misexpression suppresses Cut and stg-lacZ expression, suggesting that Hnt acts as a transcriptional repressor. Consistent with this interpretation, the mammalian homolog of Hnt, RREB1, also acts as a transcriptional repressor in several cellular contexts (Sun, 2007).

An interesting observation from these studies is that ttk clones have a phenotype similar to that of hnt clones. As in Notch regulation of Hnt, ttk is possibly downstream of Notch, but the current analysis of Notch mutants in stage-1 to stage-10 egg chambers showed no obvious change in Ttk expression. It was also found that Hnt has no role in regulating ttk expression. The findings that ttk expression is not regulated by Hnt or Notch during midoogenesis is perhaps not surprising given that Ttk69 is evenly expressed throughout early and midoogenesis. The phenotypic similarity between hnt and ttk mutants suggests that ttk and hnt act cooperatively to suppress gene expression at the M/E transition. Ttk may act as a permissive signal for Hnt to regulate Ci expression and the M/E switch. In the absence of either one, the M/E switch cannot take place properly. Consistent with this hypothesis, Ttk is known to act as a transcriptional repressor in the Drosophila eye. Whether Hnt and Ttk bind directly to the regulatory sequence of the cell-cycle genes and/or ci remains unclear (Sun, 2007).

Several lines of evidence suggest that the role of Hnt in promoting the M/E switch is not universal. First, during embryogenesis, a hnt-deficiency line enters the G1 arrest normally after cycle 16 in epidermal cells and undergoes normal M/E switch in the salivary gland, although Fzr is required for this process. Second, nurse-cell endoreplication does not require Hnt; no obvious defect was detected in hnt germline clones. The specific role of Hnt in follicle-cell-cycle regulation may stem from its role in regulating cell differentiation. For example, Hnt expression may cause upregulation of Fzr through the downregulation of Cut. This indirect role of Hnt suggests that the cell-cycle regulation may be a by-product of cell differentiation (Sun, 2007).

Both Notch and Hh signaling pathways are implicated in the regulation of differentiation and proliferation, but precisely how the two interact in regulating cellular processes is poorly understood. Depending on the cellular environment, their effects on proliferation and differentiation differ. In Drosophila eye imaginal discs, Notch triggers the onset of proliferation during the second mitotic wave (SMW), the opposite of its role in follicle-cell development. In the SMW, Notch positively affects dE2F1 and CycA expression and promotes S phase entry. In these cells, Hh signaling, along with Dpp, activates Dl expression, thereby activating the Notch pathway. Hh and Notch therefore act sequentially and positively during the SMW, whereas, in follicle cells, they act antagonistically. Hh signaling is active in the mitotic follicle cells in early oogenesis, but it is downregulated during the M/E switch when Notch signaling is activated. Notch appears to be superimposable on Hh signaling; mutation of the negative regulator of the Hh pathway, ptc, in follicle cells cannot interfere with the activation of Notch signaling as long as these cells are in direct contact with the germline cells. These ptc mutant cells show no accumulation of Ci-155, consistent with the finding that Notch signaling suppresses ci transcription through Hnt. The ptcS2 cells that were out of contact with germline cells remained in the mitotic cycle because they could not receive Dl signaling from them, suggesting that Hh signaling is sufficient to keep these cells in the undifferentiated and mitotically active state (Sun, 2007).

Notch-dependent activation of Hnt and downregulation of Ci may be involved in another follicle-cell process, the migration of a specialized group of anterior follicle cells toward the border between the nurse cells and the oocyte at stage 9. These so-called border cells showed downregulation of ci during migration. When slbo-Gal4 was used to drive Ci overexpression in border cells, ~66% of egg chambers showed defects in border-cell migration. Notch signaling, as well as ttk, has been reported to be required for border-cell migration. Hnt was found to be expressed in the border cells and depended on Notch signaling. The occasional hnt border-cell clones observed also showed defects in border-cell migration, so the crosstalk between Hh and Notch through Hnt may go beyond the regulation of the M/E switch in follicle cells (Sun, 2007).

Table of contents

Notch: Biological Overview | Evolutionary Homologs | Regulation | Protein Interactions | Post-transcriptional regulation of Notch mRNA | Developmental Biology | References

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