InteractiveFly: GeneBrief

Tie-like receptor tyrosine kinase: Biological Overview | References

Induction of cell death by a variety of means in wing imaginal discs of Drosophila larvae resulted in the activation of an anti-apoptotic microRNA, bantam. Cells in the vicinity of dying cells also become harder to kill by ionizing radiation (IR)-induced apoptosis. Both ban activation and increased protection from IR required receptor tyrosine kinase Tie, which was identified in a genetic screen for modifiers of ban. tie mutants are hypersensitive to radiation, and radiation sensitivity of tie mutants was rescued by increased ban gene dosage. It is proposed that dying cells activate ban in surviving cells through Tie to make the latter cells harder to kill, thereby preserving tissues and ensuring organism survival. The protective effect reported in this study differs from classical radiation bystander effect in which neighbors of irradiated cells become more prone to death. The protective effect also differs from the previously described effect of dying cells that results in proliferation of nearby cells in Drosophila larval discs. If conserved in mammals, a phenomenon in which dying cells make the rest harder to kill by IR could have implications for treatments that involve the sequential use of cytotoxic agents and radiation therapy (Bilak, 2014).

In metazoa where cells exist in the context of other cells, the behavior of one affects the others. The consequences of such interactions include not just cell fate choices but also life and death decisions. In wing imaginal discs of Drosophila melanogaster larvae, dying cells release mitogenic signals. Signaling from dying cells, or dying cells kept alive by the caspase inhibitor p35 (the so-called 'undead' cells), in wing discs operate through activation of Wingless (Drosophila Wnt) and JNK, and through repression of the tumor suppressor Salvador/Warts/Hippo pathway. A crosstalk between JNK and Hpo has also been reported. The consequences on the neighbors include increased number of cells in S phase and activation of targets of Yki, a transcription factor that is normally repressed by Hpo signaling. Mitogenic signals from dying cells results in increased proliferation of neighbors, which is proposed to compensate for cell loss and help regenerate the disc (Bilak, 2014).

A target of Yki is bantam microRNA, but ban was not examined in above-described studies. ban was first uncovered in a genetic screen for promoters of tissue growth when overexpressed in Drosophila. Further study found a role for ban in both preventing apoptosis and promoting proliferation. A key target of ban in apoptosis is hid, a Drosophila ortholog of mammalian SMAC/Diablo proteins. These proteins antagonize DIAP1 to liberate active caspases and allow apoptosis. Hid is pro-apoptotic; repression of Hid by ban via binding sites in hid 3′UTR curbs apoptosis (Bilak, 2014).

Since the initial characterization of ban, the role of this miRNA has expanded to include coordinating differentiation and proliferation in neural and glial lineages, cell fate decisions in germ line stem cells, in circadian rhythm, and in ecdyson hormone production. In these and other contexts, ban is regulated by a number of transcriptional factors and signaling pathways including, Hpo/Yki, Wg, Myc, Mad, Notch and Htx. The regulatory region of ban gene is likely to be complex and substantial; p-element insertions more than 10 kb away from ban sequences produce ban phenotypes (Bilak, 2014).

The experimental evidence in Drosophila that dying cells promote proliferation presaged by several years the experimental evidence for a similar but mechanistically different phenomenon in mammals. A response called 'Phoenix Rising' occurs in mice after cell killing by ionizing radiation. Here, the activity of Caspase 3 and 7 is required in dying cells and mediates the release of prostaglandin E₂, a stimulator of cell proliferation (Li, 2010). These signals act non-autonomously to stimulate proliferation and tissue regeneration. A follow-up study in mice found a requirement for Caspase 3 in tumor regeneration after radiation treatment (Huang, 2011). Not all consequences on neighboring cells are protective or mitogenic. In the classical 'radiation bystander effect', seen in cell culture and in mice, the effect of irradiated cells on the neighbors is destructive, making the latter more prone to death (Mothersill, 2006; Singh, 2011). There is evidence for a soluble signal; media from irradiated cells can induce the bystander effect on naïve cells. Inhibitors of the bystander effect include antioxidants, suggesting that oxidative stress and energy metabolism may be involved in radiation bystander effect (Bilak, 2014).

It has been shown previously that ban activity increased after exposure to ionizing radiation (IR) in wing imaginal discs of Drosophila larvae (Jaklevic, 2008). IR-induced increase in ban activity required caspase activity: expression of a viral caspase inhibitor, p35, or mutations in p53 that reduced and delayed the onset of caspase activation attenuated ban activation. It is noted that while IR-induced cell death is scattered throughout the disc, ban activation is homogeneous. This suggested a non-cell-autonomous component in activation of ban. The current study came out of efforts to understand how ban is activated in response to IR. Drosophila tie, which encodes a receptor tyrosine kinase of VGFR/PDGFR family, was identified as an important mediator of IR-induced changes in ban. Previous knowledge of Tie function in Drosophila was limited to long range signaling for border cell migration during oogenesis (Wang, 2006). This study reports that Tie was needed to activate ban in response to cell death. One consequence of ban activation was that remaining cells were harder to kill by IR (Bilak, 2014).

This study has documented a previously unknown phenomenon in wing imaginal discs of Drosophila larvae; dying cells protected nearby cells from death. Killing cells by any one of three methods -- ptc-GAL4-driven expression of dE2F1^RNAi or pro-apoptotic genes hid and rpr, exposure to ionizing radiation (IR) and clonal induction of Hid/Rpr -- activated an anti-apoptotic microRNA, bantam. Death by ptc-GAL4 or clonal expression of Hid/Rpr also made surviving cells more resistant to killing by IR. The protective effect was sensitive to ban gene dosage. This phenomenon was named 'Mahakali effect', after the Hindu goddess of death who protects her followers. Mahakali effect differs from classical radiation 'bystander effect' in which byproducts from cell corpses make surviving cells more prone to death. The Mahakali effect appears to operate in a non-cell-autonomous fashion. Disc-wide protection by ptc4>Rpr and Hid/Rpr that included even cells in the P compartment that did not express ptc, provides the strongest evidence for non-autonomy. This idea is supported by the finding that IR-induced caspase activation was reduced in cells outside Hid/Rpr flip-out clones (Bilak, 2014).

A recent paper describes a non-autonomous induction of apoptosis by apoptotic cells (Perez-Garijo, 2013). These results do not necessarily contradict what is reported in this study. Most of the experiments in the published work used undead cells kept alive by p35; Mahakali effect is seen without p35. Non-autonomous apoptosis was assayed at, typically, 3-4 days after induction of undead cells; this study detected Mahakali effect 6 hr after cell death induction using similar death-inducing stimuli (Hid/Rpr). It would be interesting to see how long Mahakali effect persists and whether non-autonomous apoptosis, occurring at longer time points, also produces Mahakali effects of its own. Another recent paper describes tissue regeneration after massive cell ablation in wing discs (Herrera, 2013). It would also be interesting to see if the Mahakali effect operates among regenerating cells (Bilak, 2014).

The data shown in this study suggest that the basic components of the Mahakali effect are caspase activity in dying cells (because expression in dying cells of p35, an inhibitor of effector caspases, blocked ban activation), ban (because ban activation resulted from cell death and the protective effect was sensitive to ban gene dosage), and tie (because tie was required to activate ban and the protective effect was sensitive to tie gene dosage). A model is proposed in which caspase activity in dying cells acts through Tie to cause non-autonomous activation of ban and the Mahakali effect. A validated target of ban in apoptosis inhibition is hid, whose 3'UTR includes 4 potential ban binding sites. Previous work has shown that a GFP sensor with hid 3'UTR is reduced after IR (Jaklevic, 2008), reflecting repression of hid by ban. Deletion of two potential ban-binding sites in the hid 3'UTR abolished the IR-induced changes in GFP (Bilak, 2014).

The Mahakali effect differs in two ways from previously described effects of dead/dying cells in wing discs. First, the Mahakali effect extended further than previously reported signaling from dead/dying cells. In the extreme case of ptc4>Hid/Rpr, the protection reached as far as the edge of the disc. This distance, on of order of 100 or more mm is comparable to the distance of border cell migration, in which Tie is known to function. In contrast, the mitogenic effect that occurs through JNK/Wingless in response to undead cells in the wing disc is seen up to 5 cells away. Activation of proliferation through the Hpo/Yki axis also spans 3-5 cells away. This can be seen as activation of Yki targets such as DIAP1. This result could be reproduced: ptc4>dE2f1^RNAi activated a Yki target, DIAP1, but only within or close to the ptc domain. Yki^B5 allele, which disrupts cell death-induced proliferation, did not alter the Mahakali effect, further supporting the idea that the two effects are different. Second, ban activation in response to cell death was sensitive to the caspase inhibitor p35. In contrast, the mitogenic effect of dying cells in wing imaginal discs is not sensitive to p35. It is noted that the mitogenic effect of dying cells is inhibited by p35 in the differentiating posterior region of eye imaginal discs, which is similar to what was seen for ban activation in the wing discs (Bilak, 2014).

This study found that tie was required for IR-induced activation of ban and for larval survival after irradiation. There were similarities as well as differences in the role of ban and tie. tie mutants were IR-sensitive, as are viable alleles of ban (Jaklevic, 2008). Tissue-specific overexpression of ban results in abnormal growth; this study found that 6 independent UAS-tie transgenic lines were lethal when driven by actin-GAL4. Thus, too much ban or tie has consequences. On the other hand, reducing tie or ban gene dosage by half attenuated the Mahakali effect. Thus, too little ban or tie also has consequences. In fact, UAS-ban or UAS-tie without a GAL4-driver was sufficient to rescue ban and tie mutant phenotypes. Thus, intermediate levels of expression may be important for the function of these genes (Bilak, 2014).

The biggest difference between ban and tie, of course, was that while tie homozygous larvae were viable (this study), ban homozygous larvae are lethal. tie became necessary only after radiation exposure. This suggests that tie was needed to regulate ban not during normal development but after radiation exposure. How is IR and cell death linked to Tie? mRNA for Pvf1, a ligand for Tie in border cell migration, was found to be induced by IR and this induction appeared to be dependent on cell death (abolished in p53 mutants). Pvf1^EP1624 mutants that are mRNA and protein null, also showed reduced Mahakali effect. The degree of reduction was significant but not back to the level seen in control discs without ptc4>dE2f1^RNAi, suggesting the involvement of additional ligands or mechanisms for Tie activation. In agreement, no ban activation or the Mahakali effect was seen after overproduction of Pvf1. Pvf1 was necessary but insufficient to produce these effects without cell death (Bilak, 2014).

Tie activated ban, at least in part by increasing ban levels. How IR and caspase activity promotes Pvf1 expression and how Tie activity increases ban levels will be key questions to address in the future. Testing the role of known apoptosis regulators, such as Diap1, and signaling molecules, such as Wg, may help address these questions. The genetic screen that identified Tie will be completed in future studies; it has the potential to identify additional components of the Mahakali effect (Bilak, 2014).

Pvr, a PDGF/VEGF receptor homolog that functions redundantly with Tie in border cell migration, also plays an anti-apoptotic role in embryonic hemocytes. A recent study in wing discs found that Pvr is activated in neighbors of dying cells in a JNK-dependent manner, to result in cytoskeletal changes that allow the engulfment of the dead cell by the neighbor (Ohsawa, 2011). It is interesting that two PDGF/VEGF receptor homologs that function redundantly in cell migration during oogenesis may also play non-redundant roles in non-autonomous responses to cell death in wing discs (Bilak, 2014).

Cancer therapy routinely comprises the application of two or more cytotoxic agents (taxol and radiation, for example) to cancer cells. A phenomenon in which cell killing by one agent influence resistance to the second agent is, therefore, of potential clinical significance. The bulk of the current analysis focused on protection from IR-induced cell death. But preliminary evidence indicates that the Mahakali effect can also protect against cell death induced by maytansinol, a microtubule depolymerizing agent with relevance to cancer therapy that we found before to induce cell death in Drosophila wing discs. An important question is whether a phenomenon like Mahakali effect exists in mammals and acts as a survival mechanism in response to cell death. Ang-1, a ligand for mammalian Tie-2, is a pro-survival factor for endothelial cells during serum deprivation and after irradiation in cell culture models (Holash, 1999; Kwak, 2000; Papapetropoulos 2000). Interestingly, Ang1 is produced not by endothelial cells but by neighbors, at least in cell culture. Based on these data, it is possible that radiation exposure results in Ang1 production by dead/dying cells that promote the survival of endothelial cells via Tie-2. Consistent, an Ang-1 derivative that is a potent activator of Tie-2 has been shown to protect endothelial cells from radiation-induced apoptosis (Cho, 2004; Bilak, 2014 and references therein).

Tie-mediated signal from apoptotic cells protects stem cells in Drosophila melanogaster

Many types of normal and cancer stem cells are resistant to killing by genotoxins, but the mechanism for this resistance is poorly understood. This study shows that adult stem cells in Drosophila melanogaster germline and midgut are resistant to ionizing radiation (IR) or chemically induced apoptosis; the mechanism for this protection was dissected. Upon IR the receptor tyrosine kinase Tie/Tie-2 is activated, leading to the upregulation of microRNA bantam that represses FOXO-mediated transcription of pro-apoptotic Smac/DIABLO orthologue, Hid in germline stem cells. Knockdown of the IR-induced putative Tie ligand, PDGF- and VEGF-related factor 1 (Pvf1)), a functional homologue of human Angiopoietin, in differentiating daughter cells renders germline stem cells sensitive to IR, suggesting that the dying daughters send a survival signal to protect their stem cells for future repopulation of the tissue. If conserved in cancer stem cells, this mechanism may provide therapeutic options for the eradication of cancer (Xing, 2015).

A form of programmed cell death, apoptosis, is characterized as controlled, caspase-induced degradation of cellular compartments to terminate the activity of the cell. Apoptosis plays a vital role in various processes including normal cell turnover, proper development and function of the immune system and embryonic development. Apoptosis is also induced by upstream signals, such as DNA double-strand breaks (DSB), to destruct severely damaged cells. DSB activate ATM checkpoint kinase and Chk2 kinase-dependent p53 phosphorylation and induction of repair genes. However, if DSB are irreparable, p53 activation will result in pro-apoptotic gene expression and cell death. However, aggressive cancers contain cells that show inability to undergo apoptosis in response to stimuli that trigger apoptosis in sensitive cells. This feature is responsible for the resistance to anticancer therapies, as well as the relapse of tumours after treatment, yet the molecular mechanism of this resistance is poorly understood (Xing, 2015).

As the cell type that constantly regenerates and gives rise to differentiated cell types in a tissue, stem cells share high similarities with cancer stem cells, including unlimited regenerative capacity and resistance to genotoxic agents. Adult stem cells in model organisms such as Drosophila melanogaster, have been utilized to study stem cell biology and for conducting drug screens, thanks to their intrinsic niche, which provides authentic in vivo microenvironment. This study shows that Drosophila adult stem cells are resistant to radiation/chemical-induced apoptosis, and the mechanism for this protection was dissected. A previously reported cell survival gene with a human homologue, pineapple eye (pie) , acts in both stem cells and in differentiating cells to repress the transcription factor FOXO. Elevated FOXO levels in pie mutants lead to apoptosis in differentiating cells, but not in stem cells, indicating the presence of an additional anti-apoptotic mechanism(s) in the latter. We show that this mechanism requires Tie, encoding a homologue of human receptor tyrosine kinase Tie-2, and its target, bantam, encoding a microRNA. The downstream effector of FOXO, Tie and ban, is show to be Hid, encoding a Smac/DIABLO orthologue. Knocking down the ligand Pvf1/PDGF/VEGF/Ang in differentiating daughter cells made stem cells more sensitive to radiation-induced apoptosis, suggesting that Pvf1 from the apoptotic differentiating daughter cells protects stem cells (Xing, 2015).

This study shows that an anti-apoptotic gene, pie, is required for stem cell self-renewal but not for resistance to apoptosis, indicating a compensatory anti-apoptotic mechanism in stem cells. The cell cycle marker profile of pie GSCs resembles that of InR deficient GSCs, leading to the finding that pie controls GSC, as well as ISC self-renewal/division through FOXO protein levels. Surprisingly, pie targets FOXO as well in differentiating cells, failing to explain why the loss of pie does not induce apoptosis in stem cells. However, while the upregulation of FOXO leads to the upregulation of its apoptotic target Hid in differentiating cells, in adult stem cells Hid is not upregulated. Hence additional regulatory pathway is in place to repress Hid and thereby apoptosis in stem cells. This study identified Tie-receptor as the key gatekeeper for the process in the GSCs. The signal (Pvf1) from the dying daughter cells activates Tie in GSCs to upregulate bantam microRNA that represses Hid, thereby protecting the stem cells. Bantam is known to repress apoptosis and activate the cell cycle. However, while protected from apoptosis in this manner, the stem cells do not activate the cell cycle but rather stay in protective quiescence through FOXO activity. When the challenge is passed, stem cells repopulate the tissue (Xing, 2015).

The mammalian pie homologue, G2E3 was reported to be an ubiquitin ligase with amino terminal catalytic PHD/RING domains. G2E3 is essential for early embryonic development (Brooks, 2008). Importantly, microarray data show significant enrichment of G2E3 expression levels in human embryonic stem (ES) cell lines. These observations suggest a critical role of G2E3 in embryonic development, potentially in maintaining the pluripotent capacity. Since FOXO is shown to be an important ESC regulator, it will be interesting to test whether defects in G2E3 result in changes in FOXO levels. Furthermore, future studies are required to test whether human ES cells also are protected from apoptosis due to external signals from dying neighbouring cells (Xing, 2015).

The cell cycle defects of pie mutant stem cells, such as abnormal cell cycle marker profile, can be a consequence of elevated FOXO levels, since FOXO is a transcription factor with wide array of target genes, many of which are involved with cell cycle progress, such as the cyclin-dependent kinase inhibitor p21/p27 (Dacapo in Drosophila). This may be critical when bantam function is considered in the stem cells. Bantam is known to function as anti-apoptotic and cell cycle inducing microRNA. While in GSC bantam is critical through its anti-apoptotic function as a Hid repressor, it has no capacity to induce GSC cell cycle after irradiation. In a challenging situation, such as irradiation, an additional protection mechanism for the tissue is to keep the stem cell in a quiescent state during challenge. bantam's pro-cell division activity may be dampened by FOXO's capacity to upregulate p21/Dacapo (Xing, 2015).

The FOXO family is involved in diverse cellular processes such as tumor suppression, stress response and metabolism. The FOXO group of human Forkhead proteins contains four members: FOXO1, FOXO3a, FOXO4, and FOXO6. Studies to elucidate their function in various stem cell types in vivo using knockout mice have shown some potential redundancy of FOXO proteins. Recent publications have demonstrated a requirement for some of the FOXO family members in mouse hematopoietic stem cell proliferation, mouse neural stem cells, leukaemia stem cells and human and mouse ES cells in vitro. However, FOXO is shown to be dispensable in the early embryonic development in mouse. Drosophila genome has only one FOXO, allowing a definitive study of FOXO's function in stem cells. This study now demonstrates that tight regulation of FOXO protein levels is essential for in vivo GSC and ISC self-renewal in Drosophila. While the loss of FOXO function generates supernumerary stem cells, inappropriately high level of FOXO results in stem cell loss. Under challenge, such as exposure to irradiation, stem cells depleted of FOXO fail to stay quiescent and become more sensitive to the damage, leading to the loss of GSC population. These data demonstrate the importance of the balanced FOXO expression level for stem cell fate (Xing, 2015).

Previous studies have shown that multiple adult stem cell types manage to avoid cell death in response to severe DNA damage. This work has studied the mechanisms that stem cells utilize to avoid apoptosis in absence of pie and revealed that apoptosis is protected through a receptor, Tie and its target miRNA bantam that can repress the pro-apoptotic gene Hid. The ligand for Tie is likely secreted from the dying neighbours since Tie is essential in GSC only after irradiation challenge, IR induces Tie's potential ligand Pvf1 expression in cystoblasts and knockdown of Pvf1 in cystoblasts eliminates stem cells' protection against apoptosis. Further studies will reveal whether the same protective pathway is utilized in other stem cells. Community phenomenon have been described previously around dying cells: compensatory proliferation, Phoenix rising, bystander effect and Mahakali. While Bystander effect describes dying cells inducing death in the neighbours, compensatory proliferation, Phoenix rising and Mahakali describe positive effects in cells neighbouring the dying cells. The present work shows that adult stem cell can survive but show no immediate induction of proliferation when neighboured by dying cells. However, since adult stem cells can repopulate the tissue when death signals have passed, it is proposed that in adult stem cells these phenomenon merge. First, the GSCs survive by bantam repressing the apoptotic inducer, Hid, and later repopulate the tissue by activating cell cycle. Recent findings have suggested that p53 might play an important role in re-entry to cell cycle in stem cells51. The results from the current studies shed light on the general understanding of stem cell behaviour in response to surrounding tissue to ensure the normal tissue homeostasis. It is also plausible that cancer stem cells hijack these normal capacities of stem cells (Xing, 2015).

Analysis of cell migration using whole-genome expression profiling of migratory cells in the Drosophila ovary

Cell migration contributes to normal development and homeostasis as well as to pathological processes such as inflammation and tumor metastasis. Previous genetic screens have revealed signaling pathways that govern follicle cell migrations in the Drosophila ovary, but few downstream targets of the critical transcriptional regulators have been identified. To characterize the gene expression profile of two migratory cell populations and identify Slbo targets, border cells and centripetal cells expressing the mouse CD8 antigen were purified and whole-genome microarray analysis was carried out. Genes predicted to control actin dynamics and the endocytic and secretory pathways were overrepresented in the migratory cell transcriptome. Mutations in five genes, including ttk, failed to complement previously isolated mutations that cause cell migration defects in mosaic clones. Functional analysis revealed a role for the Notch-activating protease Kuzbanian in border cell migration and identified Tie receptor tyrosine kinase as a guidance receptor for the border cells (Wang, 2006).

Gene expression profiling of migratory cells in the Drosophila ovary has allowed comparison of the global patterns of gene expression of developmentally regulated cell movements to that previously reported for invasive carcinoma cells. Of 30 genes that encode motility-associated proteins that were identified as upregulated in invasive breast carcinoma relative to the primary tumor, 23 have easily identified Drosophila homologs. Of these, 11 (48%) were identified as upregulated in migratory follicle cells in the current analysis. This seems noteworthy given that the cells derive from different organisms and different tissues. In contrast, only one of the cytoskeleton-associated, migratory cell-enriched genes was identified out of the top 419 genes upregulated in the adult Drosophila eye (Wang, 2006).

Finding a large number of genes that are differentially expressed in a microarray analysis can make it difficult to decide which individual genes merit additional, detailed study. One approach to limiting the number of genes in an analysis is to use stringent fold-change cutoffs. However, it is not clear that this is the best way to derive biologically meaningful information from large data sets. An alternative approach was used, employing a sensitive method to reveal a large number of differentially expressed genes and then separating the large data set into smaller sets by using gene ontology with GO Slim. This allowed discernment, in a relatively unbiased manner: genes that encode cytoskeletal proteins and proteins associated with the secretory and endosomal pathways were overrepresented in the migratory cell-enriched genes compared to the genome as a whole, providing a rationale for the selection of smaller, functionally related subsets of genes for further study (Wang, 2006).

The overrepresentation of cytoskeleton-associated gene products among the migratory cell-enriched genes is interesting to consider in light of the striking morphology of border cells during their migration. One, or occasionally two, cells at the front of the cluster extend a long dominant protrusion that can be up to 50 μm long. This may be a common morphology for cells migrating in vivo, since it has also been observed for cells of the rostral migratory stream and neural crest cells. It seems reasonable to propose that this extended morphology may require special regulation of the cytoskeleton: such regulation might differ in some respects from that of the broad, flat lamellae and ruffles formed by cells cultured on two-dimensional surfaces. For example, longer parallel bundles of F-actin are probably required to create and maintain long protrusions such as those observed in border cells (Wang, 2006).

Although the general idea that proteins associated with the cytoskeleton are important in migratory cells is not surprising, this analysis leads to generation of hypotheses regarding specific genes. For example, two proteins known to promote long, parallel actin bundles are among the migratory cell-enriched genes, including tropomyosin and fascin. Loss of function of fascin (encoded by the gene singed) does not result in a discernible border cell migration defect; however, this may be because of redundancy with tropomyosin. Similarly, loss of the filamin-like protein encoded by the cheerio locus causes a mild border cell migration defect. The microarray analysis reveals that another filamin-like protein (Jitterbug) is expressed at a higher level in the migratory follicle cell population. The microarray data therefore can guide the development of specific, testable hypotheses concerning possible gene redundancies (Wang, 2006).

In addition to proteins with well-characterized functions in actin dynamics, such as actin and actin-related proteins, a number of genes emerged from the microarray analysis that encode proteins with motifs or domains that suggest a specific role in regulating the actin cytoskeleton, but which have not yet been characterized at all. These include Rexin, a protein composed of three SH3 domains, and CG31352, which encodes a protein composed of three LIM domains and a motif resembling the villin headpiece. Mammalian homologs of these proteins exist but have not been characterized. It will therefore be of interest to determine if these genes and their products represent evolutionarily conserved, but previously unrecognized, contributors to cell motility (Wang, 2006).

Genes encoding proteins associated with the endoplasmic reticulum, Golgi apparatus, cytoplasmic vesicles, and endosomes were significantly overrepresented among the migratory cell-enriched genes compared to the genome as a whole. This observation suggests that border cells have a special need for dynamic trafficking of proteins to and from the cell surface. It has been proposed that dynamic cell-cell adhesion between border cells and nurse cells is required for the cells first to gain traction and then to translocate, and that this may involve high rates of turnover of membrane proteins such as E-cadherin. Moreover, it is clear that several receptor molecules such as Domeless, PVR, and EGFR are present at lower concentrations on the surfaces of the border cells as compared to other follicle cells. Therefore, it seems likely that there is a high rate of movement of these proteins onto and off of the plasma membrane. All of this traffic would likely require an upregulation of proteins functioning in the secretory and endocytic pathways. Consistent with this hypothesis, it has been shown that multivesicular bodies are markedly more prevalent in migrating border cells as compared to other follicle cells in the egg chamber (Wang, 2006).

Relatively little is known about the mechanisms governing centripetal cell migration. Border cells and centripetal cells take two different paths, but they arrive at the same place. Both cell types express Slbo and require E-cadherin, Rac, and myosin II for their respective movements. Thus, mechanical aspects of these two migrations may be similar. However, there are also differences in the migrations of these two cell types. Border cells completely exit the follicle cell epithelium during their migration down the center of the egg chamber. Centripetal cells, in contrast, stay connected to the outer follicle layer. In addition, the directions of the two migrations are quite different. Whereas border cells migrate posteriorly, centripetal cells migrate symmetrically toward the center, orthogonal to the path of border cell migration. Therefore, the cues that direct the two migrations must be different. Consistent with this, none of the known border cell guidance receptors is required for centripetal cell migration. In addition, the border cells and centripetal cells initiate migration at distinct times: the border cells complete their migration before the centripetal cells begin. The gene expression profile presented in this study provides a wealth of candidate genes to test for effects on centripetal cell migration and to flush out the similarities and differences between border cell and centripetal cell migration (Wang, 2006).

One goal on this study in determining the gene expression profile of border cells was to facilitate the molecular identification of genes corresponding to the mutations that cause border cell migration defects in mosaic clones. Five genes in the microarray lists, gliotactin, tramtrack, catsup, latheo, and zipper, were matched to mutant lines identified in mosaic screens. The next challenge will be to elucidate precisely how each of these genes contributes to border cell migration (Wang, 2006).

In addition to facilitating the identification of genes that cause cell migration defects in mosaic clones, the gene expression profile can identify genes that would be unlikely to be identified in such genetic screens. For example, all of the known guidance factors for border cell migration produce either no defect when mutated individually (ligands for the EGF receptor) or quite mild defects (PVF1). Their contributions become much more obvious when multiple mutations are combined. Therefore, it is a challenge to identify this class of proteins by using conventional forward genetics. In the gene expression profile reported here, an uncharacterized receptor tyrosine kinase was found to be expressed at higher levels in migratory cells and to be an Slbo target. Expression of a putatively dominant-negative form of this receptor exacerbated the migration defects associated with loss of PVR alone or loss of PVR and EGFR, implicating this receptor in guidance of border cell migration. Therefore, the expression profile has provided a source of candidate genes that would be difficult or impossible to identify by other methods (Wang, 2006).

Search PubMed for articles about Drosophila Tie

Bilak, A., Uyetake, L. and Su, T. T. (2014). Dying cells protect survivors from radiation-induced cell death in Drosophila. PLoS Genet 10: e1004220. PubMed ID: 24675716

Brooks, W. S., Helton, E. S., Banerjee, S., Venable, M., Johnson, L., Schoeb, T. R., Kesterson, R. A. and Crawford, D. F. (2008). G2E3 is a dual function ubiquitin ligase required for early embryonic development. J Biol Chem 283: 22304-22315. PubMed ID: 18511420

Cho, C. H., Kammerer, R. A., Lee, H. J., Yasunaga, K., Kim, K. T., Choi, H. H., Kim, W., Kim, S. H., Park, S. K., Lee, G. M. and Koh, G. Y. (2004). Designed angiopoietin-1 variant, COMP-Ang1, protects against radiation-induced endothelial cell apoptosis. Proc Natl Acad Sci U S A 101: 5553-5558. PubMed ID: 15060280

Herrera, S. C., Martin, R. and Morata, G. (2013). Tissue homeostasis in the wing disc of Drosophila melanogaster: immediate response to massive damage during development. PLoS Genet 9: e1003446. PubMed ID: 23633961

Holash, J., Maisonpierre, P. C., Compton, D., Boland, P., Alexander, C. R., Zagzag, D., Yancopoulos, G. D. and Wiegand, S. J. (1999). Vessel cooption, regression, and growth in tumors mediated by angiopoietins and VEGF. Science 284: 1994-1998. PubMed ID: 10373119

Huang, Q., et al. (2011). Caspase 3-mediated stimulation of tumor cell repopulation during cancer radiotherapy. Nat Med 17: 860-866. PubMed ID: 21725296

Jaklevic, B., Uyetake, L., Wichmann, A., Bilak, A., English, C. N. and Su, T. T. (2008). Modulation of ionizing radiation-induced apoptosis by bantam microRNA in Drosophila. Dev Biol 320: 122-130. PubMed ID: 18550049

Kwak, H. J., Lee, S. J., Lee, Y. H., Ryu, C. H., Koh, K. N., Choi, H. Y. and Koh, G. Y. (2000). Angiopoietin-1 inhibits irradiation- and mannitol-induced apoptosis in endothelial cells. Circulation 101: 2317-2324. PubMed ID: 10811601

Li, F., Huang, Q., Chen, J., Peng, Y., Roop, D. R., Bedford, J. S. and Li, C. Y. (2010). Apoptotic cells activate the 'phoenix rising' pathway to promote wound healing and tissue regeneration. Sci Signal 3: ra13. PubMed ID: 20179271

Mothersill, C. and Seymour, C. (2006). Radiation-induced bystander effects: evidence for an adaptive response to low dose exposures? Dose Response 4: 283-290. PubMed ID: 18648593

Ohsawa, S., Sugimura, K., Takino, K., Xu, T., Miyawaki, A. and Igaki, T. (2011). Elimination of oncogenic neighbors by JNK-mediated engulfment in Drosophila. Dev Cell 20: 315-328. PubMed ID: 21397843

Papapetropoulos, A., Fulton, D., Mahboubi, K., Kalb, R. G., O'Connor, D. S., Li, F., Altieri, D. C. and Sessa, W. C. (2000). Angiopoietin-1 inhibits endothelial cell apoptosis via the Akt/survivin pathway. J Biol Chem 275: 9102-9105. PubMed ID: 10734041

Perez-Garijo, A., Fuchs, Y. and Steller, H. (2013). Apoptotic cells can induce non-autonomous apoptosis through the TNF pathway. Elife 2: e01004. PubMed ID: 24066226

Singh, H., Saroya, R., Smith, R., Mantha, R., Guindon, L., Mitchel, R. E., Seymour, C. and Mothersill, C. (2011). Radiation induced bystander effects in mice given low doses of radiation in vivo. Dose Response 9: 225-242. PubMed ID: 21731538

Wang, X., et al. (2006). Analysis of cell migration using whole-genome expression profiling of migratory cells in the Drosophila ovary. Dev. Cell 10: 483-495. 16580993

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date revised: 20 May 2015

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