BIOLOGICAL OVERVIEW

A brief detour through one group of mammalian receptors will serve to introduce and clarify the Drosophila gene hopscotch. Mammalian cytokines, lymphokines and growth factors interact with the cytokine receptor superfamily. Despite a lack of catalytic kinase domains, these receptors combine ligand binding with the induction of tyrosine phosphorylation. This means that members of the Janus kinase (JAK) family of cytoplasmic protein tyrosine kinases physically associate with ligand-bound receptors. It is this association that results in tyrosine phosphorylation and activation.

Activated JAKs phosphorylate the receptors as well as cytoplasmic proteins belonging to a family of transcription factors called signal transducers and activators of transcription (STATs). This JAK-STAT pathway is shared by all members of the cytokine receptor superfamily. hopscotch has been identified as the Drosophila JAK homolog. marelle, or DSTAT has been identified as the fly's STAT homolog (Ihle, 1994).

The JAK-STAT pathway regulates the expression of pair rule gene even skipped early in embryogenesis. Cooperativity in a number of positive regulator mechanisms might be required to provide an appropriate level of expression of eve in certain stripes. If the function of the JAK-STAT pathway were simply to upregulate the expression of eve in specific stripes, then the level of activation provided by the HOP-STAT system will depend on the number of STAT-binding sites present in the stripe-specific enhancer regions of eve. This might hold for activation of other pair rule genes as well. If the mechanism of activation of the JAK-STAT pathway is conserved between mammals and Drosophila, then HOP should be activated by its interaction with a membrane-bound receptor lacking a kinase domain. Because Hunchback and Knirps set the anterior and posterior borders of eve stripe 3, the JAK-STAT pathwy is not needed to spatially activate eve. It is not known which, if any, receptor is required for activation of the JAK-STAT pathway early in embryogenesis, but it is clear that the pathway is established maternally (Hou, 1997).

hopscotch has been identified as one of more than 50 Drosophila oncogenes, that is, genes that cause tumors. Tumorous-lethal (Tum-l), a hopscotch mutation, causes formation of melanotic tumors and proliferative defects in larval blood cells. Tum-l is an X-linked dominant mutation that causes melanotic tumor formation and temperature sensitive lethality. The larval tissues that produce blood cells are the lymph glands, a group of small organs arranged in lobed pairs on either side of the heart (the dorsal vessel). Undifferentiated stem cells produce two classes of mature blood cells. The first class is composed of podocytes and lamellocytes, macrophage-like cells involved in encapsulation and phagocytosis of foreign objects. The second class comprise crystal cells, involved in melanization. Tum-l causes hypertrophy (enlargement) of larval lymph glands and premature differentiation of lamellocytes.

HOP can cause neoplasia (literally "new growth;" figuratively, tumors) through one of two distinct mechanisms. The first is mutational. The original hop mutation is a single nucleotide amino acid substitution in the hop gene. The second mechanism is overexpression. Overproduction of wild type HOP by fusion of the gene to a heat shock promoter and expression during the second or third instar period also results in tumor formation. Overexpression of D-raf results in a similar phenotype to overexpression of hop. Is there a link between the two? Vertebrate epidermal growth factor signaling induces the activation of JAK1. It is tempting to speculate that DER/Torpedo/EGF-R signaling, known to transduce through the ras/raf pathway, may activate HOP as well (Harrison, 1995).

The JAK/STAT pathway is required for border cell migration during Drosophila oogenesis

During Drosophila oogenesis, border cells perform a stereotypic migration. Slbo, a C/EBP transcription factor, is required for this migration. Drosophila Stat92E has been identified in a screen for gain-of-function suppressors of the slbo mutant phenotype. By clonal analysis for Stat92E and hop mutants it has been found that the JAK/STAT pathway is required in border cells for their migration. The activating ligand for the pathway, Unpaired, is expressed in polar cells. Polar cells are specialized cells that can induce border cell fate in anterior follicle cells. On its own, ectopic expression of Unpaired can induce ectopic expression of border cell markers, including Slbo. However, Stat92E mutant cells still express normal levels of Slbo protein, thus Stat92E must regulate other targets critical for border cell migration (Beccari, 2002).

Production of ectopic polar cells by exposing early egg chambers to increased Hedgehog expression appears sufficient to induce ectopic migrating border cells at stage 9. A slbo-lacZ enhancer trap is induced in extra migrating clusters at stage 9. Similar ectopic border cell clusters have been observed in egg chambers with clones of follicle cells mutant for costal2, a negative regulator of the Hedgehog signal transduction pathway. Thus the presence of polar cells, and absence of posteriorizing signal from the oocyte, may be sufficient for the induction of border cells at the appropriate developmental stage. What signals from polar cells may be responsible for induction of border cell fate in adjacent follicle cells? There is good evidence that Upd is a key signal from polar cells: Upd is specifically expressed in polar cells and acts non cell autonomously; ectopic expression of Upd induces two border cell markers; and the JAK/STAT pathway is required in border cells. Previous studies of the JAK/ STAT pathway in Drosophila have indicated that Upd expression induces Stat92E activation through the JAK kinase Hop and that the effects of Upd can be explained in this manner. Ectopic expression of Upd induces ectopic expression of Slbo. Since the JAK/STAT pathway is required in border cells and thus must be active there, Upd regulated Stat92E may normally contribute to Slbo up-regulation in border cells (Beccari, 2002).

JAK signaling is somatically required for follicle cell differentiation in Drosophila

Janus kinase (JAK) plays several signaling roles in Drosophila oogenesis. The gene for a JAK pathway ligand, unpaired, is expressed specifically in the polar follicle cells, two pairs of somatic cells at the anterior and posterior poles of the developing egg chamber. Consistent with unpaired expression, reduced JAK pathway activity results in the fusion of developing egg chambers. A primary defect of these chambers is the expansion of the polar cell population and concomitant loss of interfollicular stalk cells. These phenotypes are enhanced by reduction of unpaired activity, suggesting that Unpaired is a necessary ligand for the JAK pathway in oogenesis. Mosaic analysis of both JAK pathway transducers, hopscotch and Stat92E, reveals that JAK signaling is specifically required in the somatic follicle cells. Moreover, JAK activity is also necessary for the initial commitment of epithelial follicle cells. Many of these roles are in common with, but distinct from, the known functions of Notch signaling in oogenesis. Consistent with these data is a model in which Notch signaling determines a pool of cells to be competent to adopt stalk or polar fate, while JAK signaling assigns specific identity within that competent pool (McGregor, 2002).

The somatic cells of the ovary consist of multiple subpopulations, each with its own function(s) in the developing egg. While the germline cyst is dividing and developing within the germarium, a monolayer of somatic cells surrounds the cyst as it moves posteriorly through the germarium. As the cyst becomes enveloped by the somatic cells, the egg chamber pinches off from the germarium, entering the vitellarium. At that time, approximately 5-8 somatic cells differentiate into stalk. These flattened, disc-shaped cells are stacked together to form the spacer between successive cysts. Stalk cells connect the anterior end of a more mature egg chamber to the posterior end of the next younger chamber. Also at that time, molecular markers can distinguish the stalk cells from the polar cells, which arise from the same precursors. The polar cells are arranged as two pairs of follicle cells, one pair at either end of each chamber near the stalk cells. While the stalk cells and polar cells cease proliferation at the end of the germarium, the remaining follicle cells, which are referred to as epithelial follicle cells, divide approximately five times to expand the pool of follicle cells. Those epithelial cells later differentiate into various subpopulations with specific functions in the vitellarium. Those subpopulations are pre-patterned with mirror image symmetry along the anterior-posterior axis of the egg. Imposed on that pre-pattern, signaling from the oocyte by the TGFalpha molecule Gurken stimulates the induction of posterior polarity on the somatic cells at the posterior end. The result is an egg with coordinated polarities of the somatic and germline cells. This coordination is essential for the proper localization of maternal determinants that pattern the resulting embryo (McGregor, 2002).

Strikingly, unpaired is expressed very specifically within the ovary. After egg chambers pinch off from the germarium, upd is restricted to the two pairs of polar cells found at the anterior and posterior tips of the egg. In the germarium, upd is expressed in a cluster of somatic cells at the posterior of region 3. Presumably these are the cells that give rise to the stalk and polar cells. Expression in the polar and border cells persists until egg maturation. In situ hybridization to Stat92E RNA reveals that Drosophila STAT is expressed in both the germarium and the vitellarium. Expression in the germarium occurs in all follicle cells in region 2a and 2b; it then begins to be restricted to terminal follicle cells in region 3. In the vitellarium, Stat92E is expressed weakly at the termini of the egg chamber, but in a broader domain than only the two polar cells. After stage 9, Stat92E is strongly expressed in the nurse cells, consistent with the maternal role of Stat92E in the segmentation of the early embryo. Moreover, weak ubiquitous expression of hop is detectable in the follicular epithelium. These data are consistent with a potential role for JAK signaling in oogenesis (McGregor, 2002).

The loss of JAK pathway function in the somatic cells of the Drosophila ovary results in the fusion of adjacent cysts and/or the mislocalization of the oocyte within a cyst. Based on molecular markers for cell identity, mutations in hop or Stat92E cause the loss of stalk cells and an increase in the number of polar cells. This shift in cell fates correlates with the fusion of adjacent cysts. An allelic series of hop mutant combinations shows a range of phenotypic severity, from occasional fusion of two adjacent chambers to complete fusion of all cysts with no morphological distinction between germarium and vitellarium. The severity of the visible phenotypes is reflective of the severity of the follicle cell fate transformations. Effects on fate range from frequent appearance of one extra polar cell in the weakest mutation to consistent appearance of a dozen or more extra polar cells in more severe alleles. Phenotypes seen in mutant clones of hop and Stat92E ovaries are similar to those seen in the heteroallelic combinations of hop mutations. By using the directed mosaic technique, clone production is limited specifically to the somatic cells, thereby demonstrating that the activity of the JAK pathway is required in the follicle cells. Mosaic analysis also demonstrates that the adoption of proper epithelial cell fates requires JAK activity (McGregor, 2002).

All follicle cell subpopulations in an egg are derived from approximately three stem cells in the germarium of each ovariole. While still in the germarium, a common pool of distinct stalk and polar cell precursors is set aside from the epithelial follicle cells. Those precursors then differentiate into either stalk or polar cells. The remaining epithelial cells are pre-patterned with mirror image symmetry along the anteroposterior axis, with three distinct subpopulations at each end. The symmetry is broken at stage 6 when Gurken in the oocyte stimulates EGF receptor in the posterior terminal cells to determine posterior polarity of the egg. The three anterior terminal cell populations then become border cells, stretched (nurse cell-associated) cells, and centripetal cells. The posterior terminal cells are essential for the reorganization of the cytoskeleton in the oocyte. Those cells send an unknown signal to the germline that stimulates the reversal of microtubular polarity in the egg which is necessary for the migration of the oocyte nucleus to the anterior and for the correct localization of polarity determinants in the egg (McGregor, 2002).

Loss of JAK pathway signaling clearly influences the terminal fate of the stalk/polar cell precursors. In heteroallelic mutant combinations of hop, the number of polar cells increases while the number of stalk cells decreases. However, the sum of stalk cells plus polar cells remains approximately the same as in wild type, indicating that loss of JAK signaling is not influencing proliferation of the precursor pool, nor is it causing recruitment of epithelial follicle cells to a polar fate. This suggests a model in which the normal function of the JAK pathway is to promote the adoption of stalk cell fate in a subset of the stalk/polar cell precursor pool. JAK pathway activation may either instruct the adoption of stalk cell fates or prevent the adoption of polar cell fate. Current data do not distinguish between these alternatives (McGregor, 2002).

A second role for JAK signaling in the follicle cells was highlighted by analysis of mosaics. In chambers of the vitellarium, the immature cell marker Fas III is rapidly downregulated in all but the polar cells. However, the epithelial follicle cells do not begin to express markers of terminal differentiation until stage 7. Indeed, these cells continue to proliferate through stage 6. Nonetheless, the loss of Fas III in the epithelial cells beginning around stage 2 suggests that the identity of these cells has already begun to change. Presumably they become preliminarily committed to an epithelial follicle cell fate. In hop or Stat92E mutant clones, younger chambers retain high levels of Fas III in all the mutant cells. In more mature egg chambers (stage 7 or later) there is a consistent lack of Fas III expansion in mutant cells. The transient nature of the increase in Fas III expression suggests that the mutant cells remain in an immature state until later stages. In this model, JAK pathway activity would be necessary for the preliminary commitment step in epithelial cell differentiation that occurs after the egg chamber pinches off from the germarium. At approximately stage 7, the normal stage for terminal differentiation, the Fas III-positive JAK mutant cells lose Fas III expression, presumably because they are cued to differentiate by another signal. The consequence of loss of JAK signaling on terminal epithelial cell fates remains to be investigated (McGregor, 2002).

Several signaling pathways have been implicated in the patterning of the follicular epithelium. The best characterized are the Notch, EGFR and Hedgehog pathways. In the earliest of these activities, strong expression of hh in the terminal filament and cap cells at the anterior tip of the germarium stimulates the proliferation of the somatic stem cells. Loss of Hh signaling results in reduced follicle cell number and consequent failure to properly encapsulate the germline cyst. Recent work has demonstrated that the normal role of Hh in the ovaries is as a somatic stem cell factor and that it is necessary for the proliferation of somatic stem cells (McGregor, 2002).

After Hh activity promotes the production of a pool of follicular precursors, the stalk/polar cell precursor pool is set aside from the epithelial cell pool. The stalk/polar cell precursor pool is distinct from the epithelial pool because it ceases to proliferate as the cyst reaches the posterior end of the germarium. The method by which the stalk/polar cell precursors are determined is not known, but it has been suggested that Notch signaling, enhanced by localized Fringe activity, may be involved in the process. Similar to JAK mutants, the loss of Notch activity causes chamber fusions that are apparently the result of a failure to produce stalk cells. But unlike JAK mutants, N pathway mutants also fail to produce polar cells. Therefore, N signaling is required for the differentiation of both polar and stalk cell fates (McGregor, 2002).

So what distinguishes stalk and polar cells from one another? JAK signaling induces the adoption of stalk cell fates in a subset of the stalk/polar cell precursors. Loss of JAK pathway activity expands polar cells at the expense of stalk cells, while ectopic activation of the pathway causes a reduction of polar cells. Therefore, it is proposed that JAK pathway activity determines the terminal fate of stalk and polar cells. However, JAK activity is limited in assigning stalk cell fates to only competent cells, that is, the stalk/polar cell precursor pool. Thus, another activity, perhaps N signaling, is necessary to induce competence for stalk and polar fates. Alternatively, N signaling may be primarily responsible for the assignment of polar cell fates. One could imagine a mechanism of lateral inhibition, already linked to N signaling in various tissues, in which all the cells of the precursor pool have N activity, but that the signal becomes limited to and maintained only in the polar cells. It may be the activity of the N pathway that then drives stable expression of upd and allows the induction of stalk cell fates in neighboring cells (McGregor, 2002).

While polar and stalk cell fates are adopted as chambers exit the germarium, differentiation of the epithelial follicle cell fates is not obvious until later. At approximately stage 7, epithelial follicle cells express markers for each of the terminal identities with a clear anterior-posterior orientation. But in the absence of Grk/EGFR signaling at the posterior, a symmetrical mirror image pattern of three terminal populations of epithelial fates at each end is revealed. In wild-type ovaries, up to approximately stage 6, the oocyte signals to the overlying posterior follicle cells through Gurken. The terminal follicle cells that receive the Grk signal are induced to become posterior follicle cells. The resulting posterior follicle cells then signal to the oocyte to stimulate a cytoskeletal rearrangement. The resulting microtubular polarity drives the migration of the oocyte nucleus from the posterior to the anterior and establishes the AP axis that allows the sequestration of anterior and posterior maternal products to their respective poles. The signal from the soma for polarization of the oocyte microtubules is not yet known (McGregor, 2002).

When the developing cyst exits the germarium, there is a distinct change in the epithelial cell precursors. The level of Fas III, a marker for immature follicle cells, is rapidly reduced in all epithelial cell precursors. However, these cells do not begin to express markers for new cell identities until around stage 7. Therefore, it seems that the epithelial cells become committed to a fate early in the vitellarium, but do not terminally differentiate until later. This is consistent with the fact that the epithelial follicle cells continue to divide until stage 6. Furthermore, Grk/EGFR signaling does not impose posterior identity on epithelial cells until stage 6. So the loss of Fas III in epithelial cell precursors in the early vitellarium marks an intermediate step in specific epithelial identities. JAK signaling is involved in this step, because clones of JAK pathway mutations cause the persistence of Fas III in epithelial cell precursors in the early vitellarium. The normal loss of Fas III expression in epithelial precursors of the early vitellarium may indicate the establishment of a pre-pattern of epithelial identities determined by JAK signaling. It is attractive to speculate such a role because the secreted JAK pathway ligand Upd is expressed symmetrically at the termini of the chamber. It is easy to envision a scheme in which the strength of the Upd signal received by the epithelial cell precursors determines the ultimate epithelial identity. However, these epithelial cells would remain in a proliferative, undifferentiated program until stage 7. The event that allows terminal differentiation is unclear, but could also be a N signal, as suggested above for competence of stalk and polar cells. This is consistent with the report of a pulse of Delta protein, a N ligand, that occurs at stages 5-7. Additional work will determine whether JAK signaling is instructive for specific epithelial fates (McGregor, 2002).

Genetic analysis of contributions of dorsal group and JAK-Stat92E pathway genes to larval hemocyte concentration and the egg encapsulation response in Drosophila

Drosophila larvae defend themselves against parasitoid wasps by completely surrounding the egg with layers of specialized hemocytes called lamellocytes. Similar capsules of lamellocytes, called melanotic capsules, are also formed around 'self' tissues in larvae carrying gain-of-function mutations in Toll and hopscotch. Constitutive differentiation of lamellocytes in larvae carrying these mutations is accompanied by high concentrations of plasmatocytes, the major hemocyte class in uninfected control larvae. The relative contributions of hemocyte concentration vs. lamellocyte differentiation to wasp egg encapsulation are not known. To address this question, Leptopilina boulardi was used to infect more than a dozen strains of host larvae harboring a wide range of hemocyte densities. A significant correlation exists between hemocyte concentration and encapsulation capacity among wild-type larvae and larvae heterozygous for mutations in the Hopscotch-Stat92E and Toll-Dorsal pathways. Larvae carrying loss-of-function mutations in Hopscotch, Stat92E, or dorsal group genes exhibit significant reduction in encapsulation capacity. Larvae carrying loss-of-function mutations in dorsal group genes (including Toll and tube) have reduced hemocyte concentrations, whereas larvae deficient in Hopscotch-Stat92E signaling do not. Surprisingly, unlike hopscotch mutants, Toll and tube mutants are not compromised in their ability to generate lamellocytes. These results suggest that circulating hemocyte concentration and lamellocyte differentiation constitute two distinct physiological requirements of wasp egg encapsulation and Toll and Hopscotch proteins serve distinct roles in this process (Sorrentino, 2004).

These results suggest that the suppression of encapsulation capacity by loss of function of hop, Tl, or tub is likely to be due to distinct requirements of these genes. The suppression of lymph gland response to parasitization in the hop^M4 background is consistent with the observed reduction in hop^M4/Y encapsulation capacity and suggests that the Hopscotch protein is necessary for a parasite-induced signal for lamellocyte differentiation. This signal for lamellocyte differentiation is most likely mediated by the transcription factor Stat92E: Loss of function of one copy of Stat92E suppresses the penetrance of the hop^Tum-l-induced melanotic tumor phenotype and Stat92E is constitutively activated in Drosophila cell cultures that overexpress Hop^Tum-l. These results are consistent with the proposed Stat92E-dependent lamellocyte signal: stat92E larvae are immune compromised and are unable to mount an efficient egg encapsulation response despite exhibiting control circulating hemocyte concentration levels. Additionally, mean circulating lamellocyte percentage in hop^Tum-l/Y; stat92E^HJ/stat92E^HJ larvae that are tumor-free is ~1%, which is indistinguishable from the control value (Sorrentino, 2004).

In contrast to Hop and Stat92E, Toll and Tube appear not to play a role in lamellocyte differentiation; rather, loss-of-function mutations in Toll or tube probably suppress encapsulation via other mechanisms. Since Toll and tube larvae have very few circulating hemocytes, reduction in encapsulation in Tl and tub mutants might be due to defects in wasp egg recognition or a reduction in hemocyte proliferation that normally follows parasitization. The effect of these mutations on crystal cells is unclear. While the possibility that these mutations reduce encapsulation capacity by reducing the crystal cell population cannot be ruled out, this is unlikely, since Black cells mutant larvae without functional crystal cells are immune competent and can still successfully encapsulate wasp eggs. The fact that lymph glands of loss-of-function Tl and tub mutant larvae can support lamellocyte differentiation suggests that the low circulating hemocyte concentration in Tl and tub larvae in itself does not hinder lamellocyte differentiation or the ability of the lymph gland to disperse after the wasp egg is introduced into the hemocoel. Given that gain-of-function Tl alleles induce lamellocyte differentiation, the lack of effect of Tl^- on lamellocyte differentiation is somewhat unexpected, and it is possible that lamellocyte differentiation is in some way secondarily activated in the Tl^10b background. Thus, the wasp egg encapsulation assay is a useful tool for evaluating the genetic requirements for lamellocyte differentiation (Sorrentino, 2004).

In conclusion, this study shows that while there is substantial variation in hemocyte concentration in control larvae, this variation is consistent with a log-normal distribution. Such a distribution could be a result of the inherently logarithmic process of cell division. Using this quantitative method of circulating hemocyte concentration data analysis, it was found that previously reported circulating hemocyte concentration values for mutant larvae that exhibit reduced or increased hemocyte densities are also log-normally distributed and that approximately half of each of these mutant distributions lie beyond the limits of the control distribution, allowing ranges of circulating hemocyte concentration values to be defined as being low, control, and high. In addition, encapsulation capacity in control and DV mutant larvae correlates with circulating hemocyte concentration. Evidence for biological significance of this correlation also comes from observations that D. melanogaster larvae selected for higher resistance against A. tabida have twice as many circulating hemocytes as compared to control larvae. These observations support the notion that circulating hemocytes, possibly plasmatocytes, contribute to the efficiency of the egg encapsulation response. However, high circulating hemocyte concentration alone is insufficient to trigger encapsulation; lamellocytes must be present. For example, massive 20- to 300-fold increases in circulating hemocyte concentration involving plasmatocytes and crystal cells, but not lamellocytes, are insufficient to trigger constitutive encapsulation of self tissue in the larva. The combined use of genetic and immune approaches used in this study demonstrates that different developmental signals independently contribute to the maintenance of the steady-state hemocyte concentration in circulation and the ability to differentiate lamellocytes. Together, these physiological parameters enable larval hosts to efficiently defend themselves against wasp infections (Sorrentino, 2004).

GENE STRUCTURE

Bases in 5' UTR -620

Exons - ten

Bases in 3' UTR - 1064

PROTEIN STRUCTURE

Amino Acids - 1177

Structural Domains

HOP has homology at its carboxyl terminus to the catalytic domain of tyrosine kinases. There is a short nuclear localization signal (amino acids 315-320) (Binari, 1994.)

date revised: 22 March 2022

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