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

Howard Hughes Medical Institute Research Laboratories, Department of Embryology, Carnegie Institution, Baltimore, MD 21218, USA.
Developmental Biology (Impact Factor: 3.55). 06/2008; 317(1):59-71. DOI: 10.1016/j.ydbio.2008.02.009
Source: PubMed


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.

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    • "A role for Rab11 in proliferation and differentiation of retinal tissues has been demonstrated in Zebrafish [5]. In Drosophila Rab11 has been shown to be involved in polarization of oocytes [6,7], post-Golgi trafficking of rhodopsin [8], ommatidal formation, activation of JNK signalling during eye development [9,10] maintenance of germ line stem cells [11], embryonic nervous system development [12] and promoting terminal differentiation of follicle cells in egg chamber [13]. Rab11 has been recently shown to be involved in regulating cell-cell interaction during collective cell migration [14]. "
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    ABSTRACT: Rab11, an evolutionary conserved, ubiquitously expressed subfamily of small monomeric GTPase has been known to regulate diverse cellular and developmental events, by regulating the exocytotic and transcytotic events inside the cell. Our studies show that Rab11 regulates Drosophila adult myogenesis by controlling proliferation and differentiation of the Adult muscle precursors (AMPs). Blocking Rab11 in the AMPs, which fuse to form the Indirect Flight Muscles (IFMs) of fly, renders flies completely flightless and non-viable. The indirect flight musculature, comprising of the differentially patterned dorsal longitudinal muscles (DLMs) and dorsal ventral muscles (DVMs), is affected to different extents. Abrogating or knocking down normal Rab11 function results in severely disrupted IFMs. DLMs forming from larval templates are reduced in number along with a significant reduction in their fibre size. The de novo developing DVMs are frequently absent. The DLMs in Rab11 hypomorphs are highly reduced, showing as a small constricted mass in one half of the thorax. Further, Rab11 function is essential for growth of these muscles during later half of adult myogenesis, as down regulation of Rab11 in IFMs results in degenerated muscles and broken fibres. Finally, we show that loss of Rab11 activity in the AMPs result in acquisition of migratory characteristic of myoblast as they show cellular protrusion at their polar ends accompanied with loss of cell-cell contacts. Our data provide the first evidence of a trafficking protein playing an indispensable role in regulating early stages of adult muscle development.
    PLoS ONE 09/2013; 8(9):e73305. DOI:10.1371/journal.pone.0073305 · 3.23 Impact Factor
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    • "In addition to vesicle-related proteins, the female fusome contains recycling endosomal and lysosomal proteins [26], [27]. Therefore, the fusome has been proposed as a center of membrane recycling and a signal for the supply of ER-derived components to oocytes [26]. Furthermore, microtubule motors (e.g., Dhc and KLP61F) and microtubule-associated proteins (e.g., Lis1) are required to generate a normal female fusome structure [28], [29], [30]. "
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    ABSTRACT: 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.
    PLoS ONE 03/2013; 8(3):e58220. DOI:10.1371/journal.pone.0058220 · 3.23 Impact Factor
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    • "To better understand the function of Par-1 in the GSC centrosome orientation checkpoint, we first investigated cell cycle-dependent changes in Par-1 localization. Par-1 has been reported to localize to the spectrosome/fusome (Cox et al., 2001; Huynh et al., 2001; Lighthouse et al., 2008) (also shown in Supplementary Fig. S1), a germline-specific, endoplasmic reticulum (ER)-like organelle (Snapp et al., 2004). The spectrosome is a spherical structure that is found in GSCs, while the fusome is a branched version of the spectrosome that runs through the cytoplasm of interconnected spermatogonia at later stages (de Cuevas et al., 1997). "
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    ABSTRACT: Male germline stem cells (GSCs) in Drosophila melanogaster divide asymmetrically by orienting the mitotic spindle with respect to the niche, a microenvironment that specifies stem cell identity. The spindle orientation is prepared during interphase through stereotypical positioning of the centrosomes. We recently demonstrated that GSCs possess a checkpoint ("the centrosome orientation checkpoint") that monitors correct centrosome orientation prior to mitosis to ensure an oriented spindle and thus asymmetric outcome of the division. Here, we show that Par-1, a serine/threonine kinase that regulates polarity in many systems, is involved in this checkpoint. Par-1 shows a cell cycle-dependent localization to the spectrosome, a germline-specific, endoplasmic reticulum-like organelle. Furthermore, the localization of cyclin A, which is normally localized to the spectrosome, is perturbed in par-1 mutant GSCs. Interestingly, overexpression of mutant cyclin A that does not localize to the spectrosome and mutation in hts, a core component of the spectrosome, both lead to defects in the centrosome orientation checkpoint. We propose that the regulation of cyclin A localization via Par-1 function plays a critical role in the centrosome orientation checkpoint.
    Developmental Biology 01/2012; 361(1):57-67. DOI:10.1016/j.ydbio.2011.10.010 · 3.55 Impact Factor
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