Orbit/CLASP Is Required for Germline Cyst Formation through Its Developmental Control of Fusomes and Ring Canals in Drosophila Males

Orbit, a Drosophila ortholog of microtubule plus-end enriched protein CLASP, plays an important role in many developmental processes involved in microtubule dynamics. Previous studies have shown that Orbit is required for asymmetric stem cell division and cystocyte divisions in germline cysts and for the development of microtubule networks that interconnect oocyte and nurse cells during oogenesis. Here, we examined the cellular localization of Orbit and its role in cyst formation during spermatogenesis. In male germline stem cells, distinct localization of Orbit was first observed on the spectrosome, which is a spherical precursor of the germline-specific cytoskeleton known as the fusome. In dividing stem cells and spermatogonia, Orbit was localized around centrosomes and on kinetochores and spindle microtubules. After cytokinesis, Orbit remained localized on ring canals, which are cytoplasmic bridges between the cells. Thereafter, it was found along fusomes, extending through the ring canal toward all spermatogonia in a cyst. Fusome localization of Orbit was not affected by microtubule depolymerization. Instead, our fluorescence resonance energy transfer experiments suggested that Orbit is closely associated with F-actin, which is abundantly found in fusomes. Surprisingly, F-actin depolymerization influenced neither fusome organization nor Orbit localization on the germline-specific cytoskeleton. We revealed that two conserved regions of Orbit are required for fusome localization. Using orbit hypomorphic mutants, we showed that the protein is required for ring canal formation and for fusome elongation mediated by the interaction of newly generated fusome plugs with the pre-existing fusome. The orbit mutation also disrupted ring canal clustering, which is essential for folding of the spermatogonia after cytokinesis. Orbit accumulates around centrosomes at the onset of spermatogonial mitosis and is required for the capture of one of the duplicated centrosomes onto the fusome. Moreover, Orbit is involved in the proper orientation of spindles towards fusomes during synchronous mitosis of spermatogonial cysts.     

Fusome asymmetry and oocyte determination in Drosophila

Germline cysts containing 16 interconnected cells (cystocytes) are produced at an early stage of Drosophila oogenesis by progenitor cells known as cystoblasts that undergo four synchronous rounds of incomplete division. During cyst formation, a region of specialized, spectrin-rich cytoplasm called the fusome traverses the intercellular Connections (ring canals), linking individual cystocytes. Subsequently, 15 cystocytes begin to transport specific RNAs and other components into the remaining cell, the future oocyte. We used fusome-specific antibodies to characterize the early stages of cyst formation. During the first cystoblast division, a spherical mass of fusome material (the “spectrosome”) was associated with only one pole of the mitotic spindle, revealing that this division is asymmetric. During the subsequent three divisions, the growing fusome always associated with the pole of each mitotic spindle that remained in the mother cell, and only extended through the newly formed ring canals after each division was completed. These observations suggest that fusomes help establish a system of directional transport between cystocytes that underlies oocyte determination. © 1995 Wiley-Liss, Inc.     

The fusome mediates intercellular endoplasmic reticulum connectivity in Drosophila ovarian cysts

Drosophila ovarian cysts arise through a series of four synchronous incomplete mitotic divisions. After each round of mitosis, a membranous organelle, the fusome, grows along the cleavage furrow and the remnants of the mitotic spindle to connect all cystocytes in a cyst. The fusome is essential for the pattern and synchrony of the mitotic cyst divisions as well as oocyte differentiation. Using live cell imaging, green fluorescent protein-tagged proteins, and photobleaching techniques, we demonstrate that fusomal endomembranes are part of a single continuous endoplasmic reticulum (ER) that is shared by all cystocytes in dividing ovarian cysts. Membrane and lumenal proteins of the common ER freely and rapidly diffuse between cystocytes. The fusomal ER mediates intercellular ER connectivity by linking the cytoplasmic ER membranes of all cystocytes within a cyst. Before entry into meiosis and onset of oocyte differentiation (between region 1 and region 2A), ER continuity between cystocytes is lost. Furthermore, analyses of hts and Dhc64c mutants indicate that intercellular ER continuity within dividing ovarian cysts requires the fusome cytoskeletal component and suggest a possible role for the common ER in synchronizing mitotic cyst divisions.     

The fusome organizes the microtubule network during oocyte differentiation in Drosophila

Differentiation of the Drosophila oocyte takes place in a cyst of 16 interconnected germ cells and is dependent on a network of microtubules that becomes polarized as differentiation progresses (polarization). We have investigated how the microtubule network polarizes using a GFP-tubulin construct that allows germ-cell microtubules to be visualized with greater sensitivity than in previous studies. Unexpectedly, microtubules are seen to associate with the fusome, an asymmetric germline-specific organelle, which elaborates as cysts form and undergoes complex changes during cyst polarization. This fusome-microtubule association occurs periodically during late interphases of cyst divisions and then continuously in 16-cell cysts that have entered meiotic prophase. As meiotic cysts move through the germarium, microtubule minus ends progressively focus towards the center of the fusome, as visualized using a NOD-lacZ marker. During this same period, discrete foci rich in gamma tubulin that very probably correspond to migrating cystocyte centrosomes also associate with the fusome, first on the fusome arms and then in its center, subsequently moving into the differentiating oocyte. The fusome is required for this complex process, because microtubule network organization and polarization are disrupted in hts(1) mutant cysts, which lack fusomes. Our results suggest that the fusome, a specialized membrane-skeletal structure, which arises in early germ cells, plays a crucial role in polarizing 16-cell cysts, at least in part by interacting with microtubules and centrosomes.     

A Balbiani body and the fusome mediate mitochondrial inheritance during Drosophila oogenesis

Maternally inherited mitochondria and other cytoplasmic organelles play essential roles supporting the development of early embryos and their germ cells. Using methods that resolve individual organelles, we studied the origin of oocyte and germ plasm-associated mitochondria during Drosophila oogenesis. Mitochondria partition equally on the spindle during germline stem cell and cystocyte divisions. Subsequently, a fraction of cyst mitochondria and Golgi vesicles associates with the fusome, moves through the ring canals, and enters the oocyte in a large mass that resembles the Balbiani bodies of Xenopus, humans and diverse other species. Some mRNAs, including oskar RNA, specifically associate with the oocyte fusome and a region of the Balbiani body prior to becoming localized. Balbiani body development requires an intact fusome and microtubule cytoskeleton as it is blocked by mutations in hu-li tai shao, while egalitarian mutant follicles accumulate a large mitochondrial aggregate in all 16 cyst cells. Initially, the Balbiani body supplies virtually all the mitochondria of the oocyte, including those used to form germ plasm, because the oocyte ring canals specifically block inward mitochondrial transport until the time of nurse cell dumping. Our findings reveal new similarities between oogenesis in Drosophila and vertebrates, and support our hypothesis that developing oocytes contain specific mechanisms to ensure that germ plasm is endowed with highly functional organelles.     

Cyst geometry in the egg chambers of Calliphora erythrocephala Mg. (Diptera: Calliphoridae) ovaries

In the germarium of polytrophic ovarioles of Calliphora erythrocephala (Mg.) fly, four mitotic divisions of cystoblasts give rise to 16-cell germ-line cysts. One cell differentiates into an oocyte, while the remaining 15 cells become nurse cells. Concomitantly actin-rich ring canals are formed at the intercellular junctions. The present study considers a mutual arrangement of the ring canals formed after the second to fourth mitoses relative to the ring canal formed after the first mitotic division in different regions of the germarium and egg chambers. During the cyst formation and its movement to the posterior end of the germarium, the ring canals are displaced relative to one another, thereby giving different branching variants of the cyst. The pattern of cell interconnections becomes stable in germarium region 2b and does not change during the cyst movement along the ovariole despite the cyst polarizes and increases in size.     

Cystocyte and lymphocyte derived fusomes/spectrosomes: analogies and differences: a mini-review

Structures analogous to Drosophila spectrosomes were found in mammalian lymphocytes. Repasky and colleagues discovered an intracellular spectrin-rich structure in lymphoid cells, which had far-reaching parallels with the fusome/spectrosome of D. melanogaster germ cells. This fact implies that spectrosomes may be characteristic not only of insect germ cells, but also that an analogous structure may play an important role in other cell types. The term "spectrosome" was first used by Lin and Spradling in 1995 to describe a large sphere of fusomal material in D. melanogaster germline stem cells and their differentiated daughter cells - cytoblasts. In the D. melanogaster ovary, membrane skeletal proteins such as ankyrin, alpha/beta spectrin as well as adducin-like Hts protein(s) were found in this specific organelle - spectrosome/fusome. These orgalelles are involved in the creation of mitotic spindles and D. melanogaster cyst formation and oocyte differentiation, but the role of analogous spectrin-based aggregates found in nucleated cells still remains unclear.     

Centrosome inheritance in the male germ line of Drosophila requires hu-li tai-shao function

Cytokinesis partitions a centrosome to each daughter cell at cell division that will duplicate and assemble a bipolar spindle in the subsequent M phase. Cytokinesis is incomplete in proliferating germ cells in Drosophila and cytoplasmic channels connect sibling germ cells. Although centrosomes are essential to male fertility, the molecular mechanism that retains centrosomes in parental germ cells is not known. Cortical cytoplasmic structures known as fusomes extend through ring canals and connect cells within the cyst. Fusome assembly in males requires function of hu-li tai-shao (hts), an adducin like protein found in fusomes and in the cortical membrane cytoskeleton of somatic cells. This work used immunological and cytological methods to place hts mutants in an allelic series. Male fertile hts mutants express hts protein and generate apparently normal or fragmented fusomes. A male sterile allele does not express hts protein or show fusome structures. Gonial cells in all hts mutants showed 2 centrosomes and mitotic spindles were bipolar. Yet, primary spermatocytes, with and without fusome structures, frequently contained too many or too few centrosomes. Although spindle structures were not found in spermatocytes without centrosomes, meiotic spermatocytes with centrosomes generated bipolar, monopolar, and multipolar spindles. Collectively, these results indicate that hts function is necessary for centrosome inheritance in spermatocytes as well as for male fertility.     

The origin of asymmetry: early polarisation of the Drosophila germline cyst and oocyte

The anterior-posterior axis of Drosophila is established before fertilisation when the oocyte becomes polarised to direct the localisation of bicoid and oskar mRNAs to opposite poles of the egg. Here we review recent results that reveal that the oocyte acquires polarity much earlier than previously thought, at the time when it acquires its fate. The oocyte arises from a 16-cell germline cyst, and its selection and the initial cue for its polarisation are controlled by the asymmetric segregation of a germline specific organelle called the fusome. Several different downstream pathways then interpret this asymmetry to restrict distinct aspects of oocyte identity to this cell. Mutations in any of the six conserved Par proteins disrupt the early polarisation of the oocyte and lead to a failure to maintain its identity. Surprisingly, mutations affecting the control of the mitotic or meiotic cell cycle also lead to a failure to maintain the oocyte fate, indicating crosstalk between the nuclear and cytoplasmic events of oocyte differentiation. The early polarity of the oocyte initiates a series of reciprocal signaling events between the oocyte and the somatic follicle cells that leads to a reversal of oocyte polarity later in oogenesis, which defines the anterior-posterior axis of the embryo.     

The abnormal spindle protein is required for germ cell mitosis and oocyte differentiation during Drosophila oogenesis

In this study, we present evidence that the asp function is required in oogenesis for germline cell divisions as well as for cyst polarity and oocyte differentiation. Consistent with previously described roles in spindle organization during Drosophila meiosis and mitosis, asp mutation leads to severe defects in spindle microtubule organization within the germarium. The mitotic spindles of the mutant cystocytes are composed by wavy microtubules and have abnormal poles that often lack gamma-tubulin. The fusome structure is also compromised. In the absence of asp function, the cystocyte divisions fail resulting in egg chamber with fewer than 16 germ cells. Moreover, the microtubule network within the developing germline cysts may assemble incorrectly in turn affecting the microtubule based transport of the specific determinants that is required during mid-oogenesis for the oocyte differentiation program.     

The Ovhts polyprotein is cleaved to produce fusome and ring canal proteins required for Drosophila oogenesis

An essential component of normal development is controlling the transition from cell proliferation to differentiation. One such transition occurs during Drosophila oogenesis. In early oogenesis, germ cells undergo mitotic proliferation and contain a specialized organelle called a fusome, whereas later post-mitotic cells differentiate and lose the fusome as F-actin-rich ring canals form. The hts gene encodes the only Drosophila Adducin, and is a female-sterile mutant that affects both the fusome and ring canals. We show that one Hts protein, Ovhts, is a polyprotein that is cleaved to produce two products, Ovhts-Fus and Ovhts-RC. Whereas Ovhts-Fus localizes to the fusome in mitotic cells, Ovhts-RC localizes to ring canals throughout later oogenesis. We demonstrate that an uncleavable version of Ovhts delays the transition from fusome-containing cells to those that have ring canals. Ovhts is the first polyprotein shown to produce proteins that function in separate structures.     

Formation, architecture and polarity of female germline cyst in Xenopus

Little is known about the formation of germline cyst and the differentiation of oocyte within the cyst in vertebrates. In the majority of invertebrates in the initial stages of gametogenesis, male and female germ cells develop in full synchrony as a syncytia of interconnected cells called germline cysts (clusters, nests). Using electron microscopy, immunostaining and three-dimensional reconstruction, we were able to elucidate the process of cyst formation in the developing ovary of the vertebrate Xenopus laevis. We found that the germline cyst in Xenopus contains 16 cells that are similar in general architecture and molecular composition to the cyst in Drosophila. Nest cells are connected by cytoplasmic bridges that contain ring canal-like structures. The nest cells contain a structure similar to the Drosophila fusome that that is probably involved in anchoring of the centrioles and organization of the primary mitochondrial cloud (PMC) around the centriole. We also find that in contrast to other organisms, in Xenopus, apoptosis is a rare event within the developing ovary. Our studies indicate that the processes responsible for the formation of female germline cysts and the establishment of germ cell polarity are highly conserved between invertebrates and vertebrates. The dissimilarities between Drosophila and Xenopus and the uniqueness of each system probably evolved through modifications of the same fundamental design of the germline cyst.     

Drosophila par-1 is required for oocyte differentiation and microtubule organization

BACKGROUND: Drosophila oocyte determination involves a complex process by which a single cell within an interconnected cyst of 16 germline cells differentiates into an oocyte. This process requires the asymmetric accumulation of both specific messenger RNAs and proteins within the future oocyte as well as the proper organization of the microtubule cytoskeleton, which together with the fusome provides polarity within the developing germline cyst. RESULTS: In addition to its previously described late oogenic role in the establishment of anterior-posterior polarity and subsequent embryonic axis formation, the Drosophila par-1 gene is required very early in the germline for establishing cyst polarity and for oocyte specification. Germline clonal analyses, for which we used a protein null mutation, reveal that Drosophila par-1 (par-1) is required for the asymmetric accumulation of oocyte-specific factors as well as the proper organization of the microtubule cytoskeleton. Similarly, somatic clonal analyses indicate that par-1 is required for microtubule stabilization in follicle cells. The PAR-1 protein is localized to the fusome and ring canals within the developing germline cyst in direct contact with microtubules. Likewise, in the follicular epithelium, PAR-1 colocalizes with microtubules along the basolateral membrane. However, in either case PAR-1 localization is independent of microtubules. CONCLUSIONS: The Drosophila par-1 gene plays at least two essential roles during oogenesis; it is required early in the germline for organization of the microtubule cytoskeleton and subsequent oocyte determination, and it has a second, previously described role late in oogenesis in axis formation. In both cases, par-1 appears to exert its effects through the regulation of microtubule dynamics and/or stability, and this finding is consistent with the defined role of the mammalian PAR-1 homologs.     

New components of the Drosophila fusome suggest it plays novel roles in signaling and transport

The fusome plays an essential role in prefollicular germ cell development within insects such as Drosophila melanogaster. Alpha-spectrin and the adducin-like protein Hu-li tai shao (Hts) are required to maintain fusome integrity, synchronize asymmetric cystocyte mitoses, form interconnected 16-cell germline cysts, and specify the initial cell as the oocyte. By screening a library of protein trap lines, we identified 14 new fusome-enriched proteins, including many associated with its characteristic vesicles. Our studies reveal that fusomes change during development and contain recycling endosomal and lysosomal compartments in females but not males. A significant number of fusome components are dispensable, because genetic disruption of tropomodulin, ferritin-1 heavy chain, or scribble, does not alter fusome structure or female fertility. In contrast, rab11 is required to maintain the germline stem cells, and to maintain the vesicle content of the spectrosome, suggesting that the fusome mediates intercellular signals that depend on the recycling endosome.     

A spectraplakin is enriched on the fusome and organizes microtubules during oocyte specification in Drosophila

BACKGROUND: During Drosophila oogenesis a membranous organelle called the fusome has a key function in the establishment of oocyte fate and polarity, ultimately leading to the establishment of the major body axes of the animal. The fusome is necessary for the microtubule-driven restriction of markers of oocyte fate to the oocyte, but the mechanism by which the fusome organizes the microtubules is not known.RESULTS: We have identified the spectraplakin Short stop (Shot) as a new component of the fusome. Spectraplakins are giant cytoskeletal linker proteins, with multiple isoforms produced from each gene. Shot is the sole spectraplakin in Drosophila. The phenotype caused by the absence of Shot is not similar to that of other components of the fusome but instead is similar to the absence of the downstream components that interact with microtubules: the dynein/dynactin-complex-associated proteins Egalitarian and BicaudalD. Shot is required for the association of microtubules with the fusome and the subsequent specification of the oocyte in 16-cell cysts. Shot is also required for the concentration of centrosomes into the oocyte, a process thought to be independent of microtubules because it still occurs in the presence of microtubule depolymerizing drugs. This suggests that Shot may protect some microtubules from depolymerization and that these microtubules are sufficient for this process.CONCLUSIONS: Shot provides the missing link between the fusome and microtubules within meiotic cysts, which is essential for the establishment of the oocyte. Shot associates with the fusome and is required for microtubule organization. We suggest that it does this directly, via its microtubule binding GAS2 domain.     

Mast, a conserved microtubule-associated protein required for bipolar mitotic spindle organization

Through mutational analysis in Drosopjila we have identified the gene multiple asters (mast), which encodes a new 165 kDa protein. mast mutant neuroblasts are highly polyploid and show severe mitotic abnormalities including the formation of mono- and multi-polar spindles organized by an irregular number of microtubule-organizing centres of abnormal size and shape. The mast gene product is evolutionarily conserved since homologues were identified from yeast to man, revealing a novel protein family. Antibodies against Mast and analysis of tissue culture cells expressing an enhanced green fluorescent protein-Mast fusion protein show that during mitosis, this protein localizes to centrosomes, the mitotic spindle, centromeres and spindle midzone. Microtubule-binding assays indicate that Mast is a microtubule-associated protein displaying strong affinity for polymerized microtubules. The defects observed in the mutant alleles and the intracellular localization of the protein suggest that Mast plays an essential role in centrosome separation and organization of the bipolar mitotic spindle.     

Centrosome migration into the Drosophila oocyte is independent of BicD and egl, and of the organisation of the microtubule cytoskeleton

During early Drosophila oogenesis, one cell from a cyst of 16 germ cells is selected to become the oocyte, and accumulates oocyte-specific proteins and the centrosomes from the other 15 cells. Here we show that the microtubule cytoskeleton and the centrosomes follow the same stepwise restriction to one cell as other oocyte markers. Surprisingly, the centrosomes still localise to one cell after colcemid treatment, and in BicD and egl mutants, which abolish the localisation of all other oocyte markers and the polarisation of the microtubule cytoskeleton. In contrast, the centrosomes fail to migrate in cysts mutant for Dynein heavy chain 64C, which disrupts the fusome. Thus, centrosome migration is independent of the organisation of the microtubule cytoskeleton, and seems to depend instead on the polarity of the fusome.     

Orbit, a novel microtubule-associated protein essential for mitosis in Drosophila melanogaster

We describe a Drosophila gene, orbit, that encodes a conserved 165-kD microtubule-associated protein (MAP) with GTP binding motifs. Hypomorphic mutations in orbit lead to a maternal effect resulting in branched and bent mitotic spindles in the syncytial embryo. In the larval central nervous system, such mutants have an elevated mitotic index with some mitotic cells showing an increase in ploidy. Amorphic alleles show late lethality and greater frequencies of hyperploid mitotic cells. The presence of cells in the hypomorphic mutant in which the chromosomes can be arranged, either in a circular metaphase or an anaphase-like configuration on monopolar spindles, suggests that polyploidy arises through spindle and chromosome segregation defects rather than defects in cytokinesis. A role for the Orbit protein in regulating microtubule behavior in mitosis is suggested by its association with microtubules throughout the spindle at all mitotic stages, by its copurification with microtubules from embryonic extracts, and by the finding that the Orbit protein directly binds to MAP-free microtubules in a GTP-dependent manner.     

Centriolin,a centriole-appendage protein,regulates peripheral spindle migration and asymmetric division in mouse meiotic oocytes

Unlike somatic cells mitosis, germ cell meiosis consists of 2 consecutive rounds of division that segregate homologous chromosomes and sister chromatids, respectively. The meiotic oocyte is characterized by an absence of centrioles and asymmetric division. Centriolin is a relatively novel centriolar protein that functions in mitotic cell cycle progression and cytokinesis. Here, we explored the function of centriolin in meiosis and showed that it is localized to meiotic spindles and concentrated at the spindle poles and midbody during oocyte meiotic maturation. Unexpectedly, knockdown of centriolin in oocytes with either siRNA or Morpholino micro-injection, did not affect meiotic spindle organization, cell cycle progression, or cytokinesis (as indicated by polar body emission), but led to a failure of peripheral meiotic spindle migration, large polar body emission, and 2-cell like oocytes. These data suggest that, unlike in mitotic cells, the centriolar protein centriolin does not regulate cytokinesis, but plays an important role in regulating asymmetric division of meiotic oocytes.     

Mago Nashi, Tsunagi/Y14, and Ranshi form a complex that influences oocyte differentiation in Drosophila melanogaster

During Drosophila melanogaster oogenesis, a germline stem cell divides forming a cyst of 16 interconnected cells. One cell enters the oogenic pathway, and the remaining 15 differentiate as nurse cells. Although directed transport and localization of oocyte differentiation factors within the single cell are indispensible for selection, maintenance, and differentiation of the oocyte, the mechanisms regulating these events are poorly understood. Mago Nashi and Tsunagi/Y14, core components of the exon junction complex (a multiprotein complex assembled on spliced RNAs), are essential for restricting oocyte fate to a single cell and for localization of oskar mRNA. Here we provide evidence that Mago Nashi and Tsunagi/Y14 form an oogenic complex with Ranshi, a protein with a zinc finger-associated domain and zinc finger domains. Genetic analyses of ranshi reveal that (1) 16-cell cysts are formed, (2) two cells retain synaptonemal complexes, (3) all cells have endoreplicated DNA (as observed in nurse cells), and (4) oocyte-specific cytoplasmic markers accumulate and persist within a single cell but are not localized within the posterior pole of the presumptive oocyte. Our results indicate that Ranshi interacts with the exon junction complex to localize components essential for oocyte differentiation within the posterior pole of the presumptive oocyte.