InteractiveFly: GeneBrief

mon2: Biological Overview | References

Drosophila pole (germ) plasm contains germline and abdominal determinants. Its assembly begins with the localization and translation of oskar (osk) RNA at the oocyte posterior, to which the pole plasm must be restricted for proper embryonic development. Osk stimulates endocytosis, which in turn promotes actin remodeling to form long F-actin projections at the oocyte posterior pole. Although the endocytosis-coupled actin remodeling appears to be crucial for the pole plasm anchoring, the mechanism linking Osk-induced endocytic activity and actin remodeling is unknown. This study reports that a Golgi-endosomal protein, Mon2, acts downstream of Osk to remodel cortical actin and to anchor the pole plasm. Mon2 interacts with two actin nucleators known to be involved in osk RNA localization in the oocyte, Cappuccino (Capu) and Spire (Spir), and promotes the accumulation of the small GTPase Rho1 at the oocyte posterior. This study also found that these actin regulators are required for Osk-dependent formation of long F-actin projections and cortical anchoring of pole plasm components. It is proposed that, in response to the Osk-mediated endocytic activation, vesicle-localized Mon2 acts as a scaffold that instructs the actin-remodeling complex to form long F-actin projections. This Mon2-mediated coupling event is crucial to restrict the pole plasm to the oocyte posterior cortex (Tanaka, 2011).

In many cell types, asymmetric localization of specific RNAs and proteins is essential for exhibiting proper structure and function. These macromolecules are transported to their final destinations and anchored there. This latter step is particularly important for the long-term maintenance of cell asymmetry. A genetically tractable model for studying intracellular RNA and protein localization is the assembly of the pole (germ) plasm in Drosophila oocytes and embryos. The pole plasm is a specialized cytoplasm that contains maternal RNAs and proteins essential for germline and abdominal development. It is assembled at the posterior pole of the oocyte during oogenesis. Drosophila oogenesis is subdivided into 14 stages, with pole plasm assembly starting at stage 8. The functional pole plasm is assembled by stage 13, stably anchored at the posterior cortex of the oocyte and later inherited by the germline progenitors (pole cells) during embryogenesis (Tanaka, 2011).

Pole plasm assembly begins with the transport of oskar (osk) RNA along microtubules to the posterior pole of the oocyte. There, the osk RNA is translated, producing two isoforms, long and short Osk, by the alternate use of two in-frame translation start sites. Although short Osk shares its entire sequence with long Osk, the isoforms have distinct functions in pole plasm assembly. Downstream, short Osk recruits other pole plasm components, such as Vasa (Vas), to the oocyte posterior, presumably through direct interactions. By contrast, long Osk prevents pole plasm components from diffusing back into the cytoplasm. Intriguingly, embryonic patterning defects are caused by either the ectopic assembly of pole plasm [elicited by Osk translation at the oocyte anterior directed by the osk-bicoid (bcd) 3'UTR] or the leakage of pole plasm activity into the bulk cytoplasm (induced by overexpressing osk). Thus, the pole plasm must be anchored at the posterior cortex for proper embryonic development (Tanaka, 2011).

Short and long Osk also differ in their subcellular distributions. Short Osk is located on polar granules, specialized ribonucleoprotein aggregates in the pole plasm, and long Osk is associated with endosome surfaces. Intriguingly, the oocyte posterior, where endocytosis is increased, is highly enriched with markers of early, late and recycling endosomes (Rab5, Rab7 and Rab11, respectively). osk oocytes, however, do not maintain either the accumulation of endosomal proteins or the increased endocytic activity at the posterior. Furthermore, the ectopic expression of long Osk at the anterior pole of the oocyte results in the anterior accumulation of endosomal proteins along with increased endocytosis. Thus, long Osk regulates endocytic activity spatially within the oocyte (Tanaka, 2011).

The endocytic pathway has two separate roles in pole plasm assembly (see Tanaka, 2008). First, it is required for the sustained transport of osk RNA by maintaining microtubule alignment. For example, in oocytes lacking Rabenosyn-5 (Rbsn-5), a Rab5 effector protein essential for endocytosis, the polarity of the microtubule array is not maintained, disrupting osk RNA localization. A similar defect occurs in hypomorphic rab11 oocytes. Second, the endocytic pathway acts downstream of Osk to anchor the pole plasm components. In rbsn-5 oocytes aberrantly expressing osk at the anterior, Osk and other pole plasm components diffuse from the anterior cortex into the ooplasm, indicating that endocytic activity is essential for stably anchoring them to the cortex (Tanaka, 2011).

The endocytic pathway is thought to anchor pole plasm components by remodeling the cortical actin cytoskeleton in response to Osk. Pole plasm anchoring is sensitive to cytochalasin D, which disrupts actin dynamics, and requires several actin-binding proteins, such as Moesin, Bifocal and Homer. Osk induces long F-actin projections emanating from cortical F-actin bundles at the posterior pole of the oocyte. Ectopic F-actin projections are also induced at the anterior pole when long Osk is misexpressed at the oocyte anterior (Tanaka, 2008). However, when the endocytic pathway is disrupted, F-actin forms aggregates and diffuses into the ooplasm, along with pole plasm components (Tanaka, 2008). These observations led to the hypothesis that Osk stimulates endocytosis, which promotes actin remodeling, which in turn anchors the pole plasm components at the posterior oocyte cortex. However, the molecular mechanism linking Osk, the endocytic pathway and actin remodeling is still unknown (Tanaka, 2011).

This study has identified Mon2, a conserved Golgi/endosomal protein, as an essential factor in anchoring pole plasm components at the oocyte posterior cortex. Oocytes lacking Mon2 did not form F-actin projections in response to Osk, but neither did they exhibit obvious defects in microtubule alignment or endocytosis. It was also shown that two actin nucleators that function in osk RNA localization in the oocyte, Cappuccino (Capu) and Spire (Spir), play an essential role in a second aspect of pole plasm assembly: the Osk-dependent formation of long F-actin projections and cortical anchoring of pole plasm components. Finally, it was found that Mon2 interacts with Capu and Spir, and promotes the accumulation of the small GTPase Rho1 at the oocyte posterior. These data support a model in which Mon2 acts as a scaffold, linking Osk-induced vesicles with these actin regulators to anchor the pole plasm to the oocyte cortex (Tanaka, 2011).

To learn more about how the pole plasm is assembled and anchored during Drosophila oogenesis, a germline clone (GLC) screen was conducted for ethyl methanesulfonate-induced mutations showing the abnormal localization of GFP-Vas, a fluorescent pole plasm marker (Tanaka, 2008). In a screen targeting chromosomal arm 2L, six mutants were identified that mapped into a single lethal complementation group, which was named no anchor (noan). In wild-type oocytes, GFP-Vas was first detectable at the posterior pole at stage 9, where it remained tightly anchored, with a progressive accumulation of protein until the end of oogenesis. In the noan GLC oocyte, GFP-Vas initially localized to the oocyte posterior during stages 9-10a, but its level gradually decreased, becoming undetectable in the mature oocyte. Similarly, the localization of Staufen (Stau) and Osk at the posterior pole, which occurs prior to that of Vas, was not maintained in the noan oocytes. Although the noan oocytes developed into normal-looking mature oocytes, the eggs were fragile and did not develop. Therefore, it was not possible to analyze the effects of the loss of maternal noan activity on the formation of abdomen or germ cells in embryos. Nevertheless, these results indicated that noan mutations cause defective anchoring of pole plasm components to the posterior pole of the oocyte (Tanaka, 2011).

The genetic mapping and subsequent DNA sequencing of the noan locus revealed that all the noan alleles had a nonsense mutation in CG8683, which encodes a homolog of a budding yeast protein, Mon2p, also termed Ysl2p. noan is referred to as mon2. All the mon2 alleles showed identical defects in the posterior localization of GFP-Vas with full penetrance. As the mutation in the mon2^K388 allele was the most proximal to the translational initiation site among the six alleles identified, mon2^K388 was primarily used to characterize the mon2 phenotype (Tanaka, 2011).

Drosophila Mon2 consists of 1684 amino acids and represents a highly conserved protein among eukaryotes. It has two Armadillo (ARM) repeat domains, which are likely to mediate protein-protein interactions, and a DUF1981 domain, which is functionally uncharacterized. In budding yeasts, mon2 (ysl2) was identified as a gene whose mutation increases sensitivity to the Na⁺/H⁺ ionophore monensin (Murén, 2001), and is synthetically lethal with a mutation in ypt51, which encodes a Rab5 homolo. Yeast Mon2p (Ysl2p) forms a large protein complex on the surface of the trans-Golgi network and early endosomes, and it is proposed to act as a scaffold to regulate antero- and retrograde trafficking between the Golgi, endosomes and vacuoles (Efe, 2005; Gillingham, 2006; Jochum, 2002; Singer-Krüger, 2008; Wicky, 2004; Tanaka, 2011 and references therein).

This study found that Capu and Spir act together to form long F-actin projections and to anchor pole plasm components at the oocyte cortex, and that Mon2 is essential to these processes. Capu and Spir also regulate the timing for initiating ooplasmic streaming and microtubule array polarization in the oocyte (Qualmann, 2009). However, the polarity of microtubule arrays was not affected in mon2 oocytes. Therefore, Mon2 is not always required for Capu and Spir to function. Rather, it appears to regulate specifically these actin nucleators through the Osk-induced endocytic pathway (Tanaka, 2011).

Mon2 is required for the formation of Osk-induced long F-actin projections at the oocyte posterior. Interestingly, ectopic overexpression of Osk at the anterior pole in the mon2 oocyte induced granular, albeit faint, F-actin structures, indicating that Osk-induced actin remodeling does not totally cease in the mon2 oocyte. Ectopic Osk at the anterior of capu spir double-mutant oocytes also induced faint F-actin granules in the cytoplasm. Thus, additional, as yet uncharacterized, actin regulators appear to function in response to Osk. Notably, two actin-binding proteins, Bifocal and Homer, play redundant roles in anchoring Osk to the cortex. Although the precise roles of Bifocal and Homer in this process remain elusive, they might function independently of Mon2 (Tanaka, 2011).

Oocytes lacking Rab5 showed disrupted posterior cortical F-actin bundles, which was suppressed by the simultaneous loss of Osk. These results reconfirm that the endocytic pathway needs intact Osk function for actin remodeling (Tanaka, 2008). This study also found that the F-actin disorganization in rab5 oocytes is Mon2-dependent. Therefore, Mon2 can facilitate actin remodeling even when Rab5 is absent, but endosomal trafficking, in which Rab5 is involved, is crucial for regulating Mon2. Mammalian Rab5 is also involved in actin remodeling. For example, Rac1 GTPase, a regulator of F-actin dynamics, is activated by Rab5-dependent endocytosis, and the local activation of Rac1 on early endosomes and its subsequent recycling to the plasma membrane spatially regulate actin remodeling. Thus, local endocytic cycling provides a specific platform for actin remodeling in a wide range of cell types (Tanaka, 2011).

There is growing evidence that endosomes act as multifunctional platforms for many types of molecular machinery. Intriguingly, Mon2 is located on the Golgi and endosomes, without entirely accumulating at the oocyte posterior. It is therefore proposed that the Osk-induced stimulation of endocytic cycling at the oocyte posterior leads to the formation of specialized vesicles, which instruct a fraction of Mon2 to regulate the activity of Capu, Spir and Rho1 to form long F-actin projections from the cortex. Although the functional property of Osk-induced endocytic vesicles has yet to be ascertained, long Osk is known to associate with the surface of endosomes. Therefore, long Osk might modify endosome specificity to recruit and/or stabilize the machineries responsible for actin remodeling (Tanaka, 2011).

Oocytes lacking Mon2 can mature without morphological abnormalities, but their eggs are nonviable. Furthermore, Drosophila mon2 mutations show recessive lethality, indicating that Mon2 has additional functions in somatic cell development. It might function in regulating vesicle trafficking or protein targeting, as reported in yeasts. As vesicle trafficking is often linked with establishing and maintaining cell polarity, it is an attractive idea that Mon2 might regulate the polarity protein localization and/or mediate the signal transduction for cell polarization in somatic cells, as well as in germ cells. Supporting this idea, a Mon2 homolog in C. elegans has been implicated in the asymmetric division of epithelial stem cells (Kanamori, 2008; Tanaka, 2011 and references therein).

It has been proposed that long Osk localizes to the endosomal membrane and generates a positive-feedback loop for cortical anchoring of pole plasm components. Osk is also thought to generate another positive-feedback loop to maintain the polarity of microtubule arrays, and the process appears to be endosomal protein-dependent. Although Rbsn-5 is required for both feedback loops, Mon2 acts specifically in the loop regulating actin remodeling for pole plasm anchoring, indicating that the two feedback loops are regulated by distinct mechanisms. The endocytic pathway consists of multiple vesicle trafficking steps, including endocytosis, endosomal recycling, late-endosomal sorting and endosome-to-Golgi trafficking. Therefore, determining which steps in the endocytic pathway are used by the two Osk-dependent positive-feedback loops is an important aim for future exploration (Tanaka, 2011).

Search PubMed for articles about Drosophila Mon2

Efe, J. A., et al. (2005). Yeast Mon2p is a highly conserved protein that functions in the cytoplasm-to-vacuole transport pathway and is required for Golgi homeostasis. J. Cell Sci. 118: 4751-4764. PubMed ID: 16219684

Gillingham, A. K., et al. (2006). Mon2, a relative of large Arf exchange factors, recruits Dop1 to the Golgi apparatus. J. Biol. Chem. 281: 2273-2280. PubMed ID: 16301316

Jochum A., et al. (2002). Yeast Ysl2p, homologous to Sec7 domain guanine nucleotide exchange factors, functions in endocytosis and maintenance of vacuole integrity and interacts with the Arf-Like small GTPase Arl1p. Mol. Cell. Biol. 22: 4914-4928. PubMed ID: 1205289

Kanamori T., et al. (2008). β-Catenin asymmetry is regulated by PLA1 and retrograde traffic in C. elegans stem cell divisions. EMBO J. 27: 1647-1657. PubMed ID: 18497747

Qualmann, B. and Kessels, M. M. (2009). New players in actin polymerization - WH2-domain-containing actin nucleators. Trends Cell Biol. 19: 276-285. PubMed ID: 19406642

Singer-Krüger, B., et al. (2008). Yeast and human Ysl2p/hMon2 interact with Gga adaptors and mediate their subcellular distribution. EMBO J. 27: 1423-1435. PubMed ID: 18418388

Tanaka, T. and Nakamura, A. (2008). The endocytic pathway acts downstream of Oskar in Drosophila germ plasm assembly. Development 135: 1107-1117. PubMed ID: 18272590

Tanaka, T., Kato, Y., Matsuda, K., Hanyu-Nakamura, K. and Nakamura, A. (2011). Drosophila Mon2 couples Oskar-induced endocytosis with actin remodeling for cortical anchorage of the germ plasm. Development 138(12): 2523-32. PubMed ID: 21610029

Wicky, S., Schwarz, H. and Singer-Krüger B. (2004). Molecular interactions of yeast Neo1p, an essential member of the Drs2 family of aminophospholipid translocases, and its role in membrane trafficking within the endomembrane system. Mol. Cell. Biol. 24: 7402-7418. PubMed ID: 15314152

date revised: 12 October 2012

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