InteractiveFly: GeneBrief

drongo: Biological Overview | References

Collective cell migration is involved in various developmental and pathological processes, including the dissemination of various cancer cells. During Drosophila melanogaster oogenesis, a group of cells called border cells migrate collectively toward the oocyte. This study shows that members of the Arf family of small GTPases and some of their regulators are required for normal border cell migration. Notably, it was found that the ArfGAP Drongo and its GTPase-activating function are essential for the initial detachment of the border cell cluster from the basal lamina. Drongo controls the localization of the myosin phosphatase Flapwing in order to regulate myosin II activity at the back of the cluster. Moreover, toward the class III Arf, Drongo acts antagonistically to the guanine exchange factor Steppke. Overall, this work describes a mechanistic pathway that promotes the local actomyosin contractility necessary for border cell detachment (Zeledon, 2019).

Cell migration requires the precise spatiotemporal control of various determinants. In particular, motility-driving forces require the coordination of both actomyosin contractility, to generate traction forces in protrusions, and propulsive forces at the back of the cell. This spatiotemporal control is even more complex during collective cell migrations in which cells maintain contacts while migrating. Indeed, in these particular migrations, these processes need to be coordinated across the group of migrating cells. Border cell migration in the Drosophila ovary is a powerful model to investigate the mechanisms that regulate collective cell migration. Border cells (BCs) detach from the follicle epithelium surrounding the egg chambers and form a small cluster of six to ten cells that migrates invasively between the giant nurse cells that compose the center of the egg chamber, toward the oocyte. Border cells use E-cadherin to maintain cluster cohesion as well as to interact with the nurse cells. Their migration is guided by receptor tyrosine kinase (RTK) ligands that are secreted by the oocyte. During border cell migration, vesicular trafficking regulators have been involved in regulating the localization of E-cadherin between border cells, in the maintenance of active RTKs at the leading edge of the cluster, and in a cell-cell communication mechanism that restrains protrusive ability to the leader cell (Zeledon, 2019).

Vesicular transport is thus critical for the spatiotemporal control of migration determinants during border cell migration. Although previous work has focused mainly on the role of small GTPases of the Rab family, the role of Arf GTPases and their regulators during border cell migration is unknown (Zeledon, 2019).

Arf GTPases regulate the formation of vesicular transport intermediates by interacting with coatomers to bend the membrane of the donor compartment. They are grouped in three classes on the basis of amino acid similarity. Although mammals have multiple class I and class II Arfs, Drosophila possess only one Arf per class: Arf79F (class I), Arf102F (class II), and Arf51F (class III). Furthermore, another small GTPase, Sar1, is structurally related to Arfs and has also been involved in vesicular transport. In addition, there are Arl (Arf-like) proteins that are closely related to Arfs but have diverse functions. The regulation of Arfs and Arls is similar to that of other small GTPases: GDP/GTP exchange factors (GEFs) promotes their activation, while GTPase-activating proteins (GAPs) are responsible for their inactivation (Zeledon, 2019).

Class I and II Arfs and Sar1 are involved mainly in bidirectional transport within the secretory pathway. However, both class I and class II Arfs can also promote trafficking steps in the endocytic pathway. The single class III Arf (ARF6 in mammals) is present at the plasma membrane and in endosomes, where it regulates recycling to the plasma membrane (Zeledon, 2019).

Arf proteins regulate cell migration in various contexts. Notably, ARF6 regulates the recycling of integrins from dissociating focal adhesions to nascent one at the leading edge and the transport of active Rac to the plasma membrane. In mammals, a class I Arf (ARF1) regulates the formation of motile structures such as podosomes and generates actomyosin contractility by acting on different RhoGTPase. Intriguingly, these functions might be independent of the role of ARF1 in vesicular transport. Similarly, Arf regulators, in particular ArfGAPs, regulate cell migration independently of vesicular transport (Zeledon, 2019).

In Drosophila, Arf79F is required for lamellipodia formation in S2R+ cells, independently of Rac, and also in epithelial tube expansion. In the latter, the GEF Gartenzwerg (Garz) and the GAP ArfGAP1 regulate its activity. The sole member of the cytohesin family of GEFs in flies, Steppke (Step), regulates actomyosin contractility during dorsal closure. Interestingly, Step might act on both class I and III Arfs in this process (Zeledon, 2019).

To improve understanding of the vesicular machinery regulating border cell migration, an RNAi screen was performed targeting Arfs, Arls, and their potential regulators. Depletion of class I and II Arfs induced strong pleiotropic effects, while neither the expression of double-stranded RNAs (dsRNAs) against class III Arf nor against Arf-like proteins induced significant border cell migration delays. Furthermore, it was found that the depletion of several Arf regulators induced migration defects. This study focused on the ArfGAP Drongo, as it seemed to specifically affect the detachment of the border cell cluster at the onset of migration. Drongo is the ortholog of mammalian AGFG1 and was shown to be required for normal egg chamber development (Zeledon, 2019).

Drongo was found to inactivate the class III Arf at the back of the border cell cluster at the onset of border cell migration. This leads to a local decrease in the levels of myosin phosphatase and a subsequent increase in myosin II activity. In turn, this promotes contractility and allows the detachment of the border cluster from the follicle epithelium. Interestingly, it was found that Drongo acts in opposition to Step. Furthermore, it was found that this pathway seems to act independently of the kinase Par-1, which was shown to inactivate myosin phosphatase at the back of the border cell clusters. Overall, this work identifies Drongo as part of a molecular cascade promoting local actomyosin contractility by clearing the back of the cluster of the myosin phosphatase (Zeledon, 2019).

This study has shown that Arfs and several of their regulators are required for border cell migration. Although RNAi lines against Arf-like proteins and numerous regulators did not induce a phenotype, it cannot be concluded that they are not involved in border cell migration, as this study has not tested the efficiency of depletion in border cell of each potential false negatives. Downregulation of either class I or class II Arfs or of the GEF Garz in border cells induces pleiotropic effects, making it difficult to ascertain their specific role in border cell migration. However, it was found that Drongo has a specific function at the initiation of border cell migration, which requires its ArfGAP activity (Zeledon, 2019).

The results indicate that Drongo regulates contractility at the back of the cluster by controlling the localization of the myosin phosphatase. In the absence of Drongo, myosin phosphatase levels increase at the back of the cluster and consequently reduce the activity of myosin II, which is required for the detachment of the cluster. Furthermore, Drongo localizes at the trailing edge at the time of detachment, suggesting a direct role in the removal of myosin phosphatase from the back of the cluster (Zeledon, 2019).

In addition to its role in regulating myosin phosphatase at the back of the cluster during detachment, Drongo might be involved in the migration process per se. Indeed, it was found in the rescue experiments that when the Drosophila melanogaster form of Drongo that is still targeted by the interfering RNA was expressed the activity of myosin II is restored at the back of the cluster, but the migration of border cell is still incomplete. Interestingly, Drongo colocalizes partially with active myosin II (p-Sqh) both at detachment and during the migration of border cells (Zeledon, 2019).

The mechanisms by which Drongo acts on myosin phosphatase are not clear. Previous work showed that the detachment of the border cell cluster requires Notch signaling and that the polarity protein Par-1 regulates myosin phosphatase activity through the direct phosphorylation of Mbs by Par-1 (Zeledon, 2019).

Drongo depletion has no effects on Par-1 and Par-3 and it does not regulate Notch activity. As Par-1 acts directly on Mbs, it is concluded that Drongo acts in parallel to Par-1 and Par-3 by regulating the localization of myosin phosphatase. These results also indicate that Drongo is not acting upstream of Notch. It could be interesting to determine if Notch regulates drongo expression. Indeed, border cells express higher levels of drongo compared with the rest of the follicle cells (Borghese, 2006), and its human ortholog AGFG1 was found to be a direct transcriptional target of Notch1 in T cell acute lymphoblastic leukemia. Hence, drongo might be part of a subset of genes regulated by Notch to ensure the detachment of the border cell cluster (Zeledon, 2019).

Several ArfGAPs have been described as regulators of the actin cytoskeleton. Some act through direct binding of actin regulators and effectors, independently of their GAP activity. For example, ASAP1 directly interact with non-muscle myosin IIA to promote cell migration (Chen, 2016). The current results indicate that the ArfGAP activity of Drongo toward Arf51F is required for border cell migration. Furthermore, it was found that Drongo functions in opposition to the ArfGEF Step. Thus, Drongo might promote contractility by maintaining Arf51F in an inactive state to keep the rear of the cluster free of myosin phosphatase. Interestingly, Step was shown to inhibit actomyosin contractility during developmental cellularization and dorsal closure. It would be interesting to determine if Drongo could also counterbalance the activity of Step to regulate contractility during these two processes (Zeledon, 2019).

The way in which the balance between Drongo and Step regulates the localization of myosin phosphatase remains unknown. As Arf51F is involved, it is appealing to hypothesize that a specific vesicular transport event might regulate the localization of myosin phosphatase. However, no evidence was obtained that myosin phosphatase is transported to or cleared from the back of the cluster through vesicular trafficking. In particular, Mbs localized in vesicular structure was not observed, neither in control conditions nor after depletion of Drongo. Still, this does not rule out that trafficking might regulate myosin phosphatase. Indeed, this study might have overlooked the trafficking of the myosin phosphatase subunits because of technical limitations. Alternatively, it is possible that a regulator of myosin phosphatase activity is trafficked or that Arf51F might directly recruit the myosin phosphatase or a regulator of myosin phosphatase. In both cases, such a regulator is probably different than the polarity proteins Par-1 and Par-3, as their distribution was found to be unaltered after Drongo depletion. Finally, Drongo and Arf51F might remodel the protein or lipid content of the plasma membrane to allow the recruitment of Mbs. For example, ARF6, the mammalian ortholog of Arf51F, has the ability to modify the lipid composition of membranes. Further work will be necessary to discriminate among these possibilities. For example, it would be possible to try to determine if perturbing various vesicular trafficking steps by independent means affects detachment and contractility. Alternatively, it would be interesting to analyze the interactome of Arf51F in its active and inactive forms to determine if active Arf51F may directly recruit the myosin phosphatase or one of its regulators (Zeledon, 2019).

The temporally controlled expression of Drongo, the fruit fly homolog of AGFG1, is achieved in female germline cells via P-bodies and its localization requires functional Rab11

To achieve proper RNA transport and localization, RNA viruses exploit cellular vesicular trafficking pathways. AGFG1, a host protein essential for HIV-1 and Influenza A replication, has been shown to mediate release of intron-containing viral RNAs from the perinuclear region. It is still unknown what its precise role in this release is, or whether AGFG1 also participates in cytoplasmic transport. This study reports for the first time the expression patterns during oogenesis for the Arf GTPase activating protein Drongo, the fruit fly homolog of AGFG1. Temporally controlled Drongo expression is achieved by translational repression of drongo mRNA within P-bodies. This study shows a first link between the recycling endosome pathway and Drongo and finds that proper Drongo localization at the oocyte's cortex during mid-oogenesis requires functional Rab11 (Catrina, 2016).

drongo mRNA presents a spatially and temporally controlled distribution during oogenesis, which in fixed egg chambers is sensitive to denaturing protocols. After transcription in the nurse cells, drongo mRNA is transported and localized mainly at the oocyte's cortex as early as stage 7. These processes appear to be dependent on the proper organization of the microtubule filaments, as drongo mRNA does not localize properly in armi mutants, where microtubule network polarization is compromised. The drongo gene was identified using enhancer trapping, technique employed when functional redundancy may exist and especially when mutant phenotypes are very mild. Attempts to isolate a drongo mutant by EMS mutagenesis failed. By comparison with AGFG1, it is likely that drongo mutants or even a drongo-null may not exhibit severe defects during oogenesis, but it is also possible that mammalian cells express redundant factors that are not encoded in the fruit fly genome. Therefore, given the current lack of a drongo RNA-null fly stock, future studies are needed to determine whether drongo mRNA localization has functional significance during oogenesis and embryogenesis, for processes such as maternal loading. However, it is proposed that drongo mRNA transport and localization to the oocyte, although highly transient, is required for the correct timing of its translation at peak (Catrina, 2016).

drongo mRNA accumulates in the oocyte early in development, but Drongo peak expression does not occur before stage 8. This is achieved by translational repression via P-bodies of the drongo transcript before mid-ogenesis and this control continues in later stages of development. Additional support for translational control of drongo gene expression comes from the observation that similar to the reported premature expression of Osk in armi mutant egg chambers, clumps of Drongo are formed in stage >6 oocytes. These clumps resemble the Drongo-EGFP aggregations were observed at the oocyte's anterior in early stages of oogenesis and they are likely due to increased amounts of endogenous Drongo. However, when Drongo-EGFP is expressed, the EGFP tag may also facilitate aggregation (Catrina, 2016).

Although its transcript lacks UTRs, Drongo-EGFP properly localizes in the oocyte. It is possible that endogenous Drongo expression is sufficient for Drongo-EGFP's correct localization. This is supported by the increase in cortical localization of Drongo-EGFP after stage 6-7, when endogenous Drongo levels naturally begin to increase at the oocyte's cortex. However, other cellular factors could also be responsible for recruiting Drongo (Catrina, 2016).

Ectopic F-actin clumps have been previously reported for oocytes of rab germline clones, where F-actin is not properly released from the early endosomes, following endocytosis. This study found that the localization of Rab5 and Rab11 in the armi mutant is similar to their localization in wild type egg chambers. Therefore, the ectopic Drongo clumps observed in armi mutant are not due to Rab5-induced or Rab11-induced defects. Taken together, these results suggest that drongo mRNA localization is not required for the Drongo protein to reach the oocyte's cortex, which may be facilitated by the slow cytoplasmic flow (Catrina, 2016).

The clathrin triskelion is composed of three Clathrin heavy chains (Chc) and three light chains (Clc). In Drosophila egg chambers, apart from its role in the formation of clathrin-coated vesicles, Chc has been reported to play a role in the polarization of the microtubule network at stage 6. Clc has been shown to direct endocytosis by providing direction for membrane internalization. After stage 7, Drongo localizes mainly at the oocyte's cortex, where it shows significant overlap with Clc-GFP, concurrent with the activation of bulk endocytosis. Close analysis and comparison of the Drongo and AGFG1 protein sequences reveals that they both contain AP2 binding/sorting motifs (DxF, YxxΦ, where Phi;=hydrophobic residue). Drongo's tight association with Clc-GFP at the oocyte's cortex, as well as its AP2 binding motifs, suggest that it is involved in endocytosis. The fact that overexpression of Drongo-EGFP only mildly affects general endocytosis during oogenesis is not surprising, as its mammalian homolog, AGFG1, was reported to mediate endocytosis of only select cargo. However, this does not exclude a role for Drongo in endocytosis and it is possible that a lack of Drongo will have a more pronounced effect (Catrina, 2016).

Drongo directly interacts with components of the Arp2/3 complex, and Drongo's cortical colocalization with Arpc3A is seen in the oocyte. Moreover, Drongo distribution near the cortex of stage 8 oocytes becomes more diffuse when Arp2/3-dependent actin branching is impaired, suggesting that F-actin is important for Drongo's tight cortical localization at mid-oogenesis. CK-666 has been reported to interfere with formation of new Arp2/3-dependent branches without affecting existing branches or the ends of microfilaments. Therefore, it is not surprising that following drug treatment, only subtle changes is seen in the cortical F-actin network at mid-oogenesis. It is possible that Drongo-EGFP localization is mildly affected by drug-induced local perturbations in F-actin organization. Even though Drongo distribution is not affected by downregulation of Arpc4 expression, it is likely that Arp2/3 and the microfilaments play an essential role in Drongo localization during oogenesis. It is possible that Arp2/3 still shows some functionality despite Arpc4 depletion (Catrina, 2016).

Although Drongo colocalizes with Arf51F, in vitro studies are needed to determine if Drongo acts as a GAP effector for Arf51F. In the current studies the decrease in FM 4-64x uptake when Drongo is overexpressed is consistent with inactivation of Arf51F by excess of GAP activity, similar to the reduction in transferrin uptake observed in HeLa cells when SMAP1 is overexpressed (GAP effector for Arf6, which is the mammalian homolog for Arf51F56). Taken together, these results indicate that Drongo may participate in endocytosis and vesicular transport, where F-actin remodeling plays an important role (Catrina, 2016).

Reports that Drongo's mammalian homolog, AGFG1, and Rab11 are both essential for Influenza A virus replication led to an analysis of Drongo and Rab11 localization. Rab11 is required early in egg chamber development for oocyte determination. During mid-oogenesis it is required for polarized endocytic recycling toward the posterior pole of the oocyte, and for the posterior localization of AP-2α. Lack of Rab11, or reduced Drongo levels in the germarium, lead to defects in oocyte specification or an additional round of cystoblast division, respectively. Therefore, it is likely that Rab11 acts upstream of Drongo during early oogenesis, and this is supported by the observation that Rab11 appears unaffected in drongoiHMJ oocytes. However, since properly localized Drongo is still observed in drongoiHMJ oocytes at mid-oogenesis, it cannot be ruled out that Rab11 localization and function at this stage may be affected in a drongo-null egg chamber (Catrina, 2016).

Rab11 is also essential for efficient osk mRNA transport, localization and anchoring, as well as osk translation. The disruption of osk mRNA trafficking and anchoring is believed to be a consequence of the failure to posteriorly concentrate the plus ends of the microtubule network at mid-oogenesis in oocytes where Rab11 function is affected. Rab11 directly interacts with Nuf (Nuclear-fallout), a unique Arf effector that contains a conserved Rab11-binding motif. Rab11 and Nuf are both required for actin recruitment during metaphase furrow formation in embryogenesis (Catrina, 2016).

It was proposed that proteins associated with recycling endosome activity (e.g., Rab11 and Nuf) are also required for delivery of actin-remodeling proteins to the plasma membrane, such as Rac1 (Rho GTPase). Moreover, Arf51F regulates endosomal recycling and cortical actin remodeling, as well as trafficking of Rac1 to the plasma membrane. In addition, when coexpressed with Rab11DN, Drongo-EGFP particles show premature streaming, which is characteristic of actin-related mutations (e.g., capu and spire) (Catrina, 2016).

Due to their size, vesicles cannot rely solely on diffusion to reach their destination; their transport is composed of an active, directed movement on the microtubules and a passive, diffusive stage. MSD analysis reports on the overall movement for all tracks, determined for particles of various sizes and for long-range transport, and it yields constrained diffusion for Drongo-EGFP and Clc-GFP oocytes. Given the predominantly passive nature of the long-range transport of Drongo-EGFP and Clc-GFP that was observed in a wild type background, it is impressive to observe the transition to a more directional movement that occurs in Rab11DN egg chambers (Catrina, 2016).

It is propose that Drongo participates in recycling endosome trafficking and this study provides evidence that Drongo is associated with intracellular trafficking where F-actin remodeling plays an important role. The results improve understanding of AGFG1's essential role in viral RNA transport. It is possible that viruses exploit the versatility of AGFG1 to achieve efficient RNA release from the perinuclear region and to facilitate their cytoplasmic transport, thereby modulating desired changes to the cytoskeleton and to trafficking pathways. Moreover, the multicellular model system this study uses is uniquely qualified to precisely identify the steps where Drongo actively participates in the cytoplasmic transport of intron-containing viral RNAs. Elucidating Drongo's cellular role(s) will help determine why its mammalian homolog is required for transport and localization of intron-containing viral RNAs. This should open new avenues for overcoming rapid viral adaptability to drug-pressure by targeting conserved interactions between viral proteins and host cofactors, which are nonessential for cellular viability (Catrina, 2016).

Search PubMed for articles about Drosophila Drongo

Borghese, L., Fletcher, G., Mathieu, J., Atzberger, A., Eades, W. C., Cagan, R. L. and Rorth, P. (2006). Systematic analysis of the transcriptional switch inducing migration of border cells. Dev Cell 10(4): 497-508. PubMed ID: 16580994

Catrina, I. E., Bayer, L. V., Yanez, G., McLaughlin, J. M., Malaczek, K., Bagaeva, E., Marras, S. A. and Bratu, D. P. (2016). The temporally controlled expression of Drongo, the fruit fly homolog of AGFG1, is achieved in female germline cells via P-bodies and its localization requires functional Rab11. RNA Biol: 1-16. PubMed ID: 27654348

Chen, P. W., Jian, X., Heissler, S. M., Le, K., Luo, R., Jenkins, L. M., Nagy, A., Moss, J., Sellers, J. R. and Randazzo, P. A. (2016). The Arf GTPase-activating protein, ASAP1, binds nonmuscle myosin 2A to control remodeling of the actomyosin network. J Biol Chem 291(14): 7517-7526. PubMed ID: 26893376

Gasilina, A., Vitali, T., Luo, R., Jian, X. and Randazzo, P. A. (2019). The ArfGAP ASAP1 controls actin stress fiber organization via its N-BAR domain. iScience 22: 166-180. PubMed ID: 31785555

Roy, N. S., Jian, X., Soubias, O., Zhai, P., Hall, J. R., Dagher, J. N., Coussens, N. P., Jenkins, L. M., Luo, R., Akpan, I. O., Hall, M. D., Byrd, R. A., Yohe, M. E. and Randazzo, P. A. (2019). Interaction of the N terminus of ADP-ribosylation factor with the PH domain of the GTPase-activating protein ASAP1 requires phosphatidylinositol 4,5-bisphosphate. J Biol Chem 294(46): 17354-17370. PubMed ID: 31591270

Schreiber, C., Saraswati, S., Harkins, S., Gruber, A., Cremers, N., Thiele, W., Rothley, M., Plaumann, D., Korn, C., Armant, O., Augustin, H. G. and Sleeman, J. P. (2019). Loss of ASAP1 in mice impairs adipogenic and osteogenic differentiation of mesenchymal progenitor cells through dysregulation of FAK/Src and AKT signaling. PLoS Genet 15(6): e1008216. PubMed ID: 31246957

Zeledon, C., Sun, X., Plutoni, C. and Emery, G. (2019). The ArfGAP Drongo promotes actomyosin contractility during collective cell migration by releasing Myosin phosphatase from the trailing edge. Cell Rep 28(12): 3238-3248. PubMed ID: 31533044

date revised: 12 March 2020

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