Ultrastructural analysis of myoblast fusion in Drosophila

Preparation of Drosophila Tissues and Organs for Transmission Electron Microscopy

Transmission electron microscopy (TEM) is the method of choice to image the ultrastructure of cells or tissues. TEM allows the visualization of molecular complexes up to an atomic resolution. Thus, TEM data have led to important conclusions about cellular processes and supported findings obtained by functional analyses. In this chapter, we describe the preparation of Drosophila tissues for TEM and provide reliable step-by-step protocols for applying classical chemical fixation or high-pressure freezing-freeze substitution (HPF-FS) to preserve cellular structures.

Dynamin regulates the dynamics and mechanical strength of the actin cytoskeleton as a multifilament actin-bundling protein

The dynamin GTPase is known to bundle actin filaments, but the underlying molecular mechanism and physiological relevance remain unclear. Our genetic analyses revealed a function of dynamin in propelling invasive membrane protrusions during myoblast fusion in vivo. Using biochemistry, total internal reflection fluorescence microscopy, electron microscopy and cryo-electron tomography, we show that dynamin bundles actin while forming a helical structure. At its full capacity, each dynamin helix captures 12-16 actin filaments on the outer rim of the helix. GTP hydrolysis by dynamin triggers disassembly of fully assembled dynamin helices, releasing free dynamin dimers/tetramers and facilitating Arp2/3-mediated branched actin polymerization. The assembly/disassembly cycles of dynamin promote continuous actin bundling to generate mechanically stiff actin super-bundles. Super-resolution and immunogold platinum replica electron microscopy revealed dynamin along actin bundles at the fusogenic synapse. These findings implicate dynamin as a unique multifilament actin-bundling protein that regulates the dynamics and mechanical strength of the actin cytoskeletal network.

Spectrin is a mechanoresponsive protein shaping fusogenic synapse architecture during myoblast fusion

Spectrin is a membrane skeletal protein best known for its structural role in maintaining cell shape and protecting cells from mechanical damage. Here, we report that α/βH-spectrin (βH is also called karst) dynamically accumulates and dissolves at the fusogenic synapse between fusing Drosophila muscle cells, where an attacking fusion partner invades its receiving partner with actin-propelled protrusions to promote cell fusion. Using genetics, cell biology, biophysics and mathematical modelling, we demonstrate that spectrin exhibits a mechanosensitive accumulation in response to shear deformation, which is highly elevated at the fusogenic synapse. The transiently accumulated spectrin network functions as a cellular fence to restrict the diffusion of cell-adhesion molecules and a cellular sieve to constrict the invasive protrusions, thereby increasing the mechanical tension of the fusogenic synapse to promote cell membrane fusion. Our study reveals a function of spectrin as a mechanoresponsive protein and has general implications for understanding spectrin function in dynamic cellular processes.

Intracellular lumen formation in Drosophila proceeds via a novel subcellular compartment

Cellular tubes have diverse morphologies, including multicellular, unicellular and subcellular architectures. Subcellular tubes are found prominently within the vertebrate vasculature, the insect breathing system and the nematode excretory apparatus, but how such tubes form is poorly understood. To characterize the cellular mechanisms of subcellular tube formation, we have refined methods of high pressure freezing/freeze substitution to prepare Drosophila larvae for transmission electron microscopic (TEM) analysis. Using our methods, we have found that subcellular tube formation may proceed through a previously undescribed multimembrane intermediate composed of vesicles bound within a novel subcellular compartment. We have also developed correlative light/TEM procedures to identify labeled cells in TEM-fixed larval samples. Using this technique, we have found that Vacuolar ATPase (V-ATPase) and the V-ATPase regulator Rabconnectin-3 are required for subcellular tube formation, probably in a step resolving the intermediate compartment into a mature lumen. In general, our ultrastructural analysis methods could be useful for a wide range of cellular investigations in Drosophila larvae.

Mechanical tension drives cell membrane fusion

Membrane fusion is an energy-consuming process that requires tight juxtaposition of two lipid bilayers. Little is known about how cells overcome energy barriers to bring their membranes together for fusion. Previously, we have shown that cell-cell fusion is an asymmetric process in which an "attacking" cell drills finger-like protrusions into the "receiving" cell to promote cell fusion. Here, we show that the receiving cell mounts a Myosin II (MyoII)-mediated mechanosensory response to its invasive fusion partner. MyoII acts as a mechanosensor, which directs its force-induced recruitment to the fusion site, and the mechanosensory response of MyoII is amplified by chemical signaling initiated by cell adhesion molecules. The accumulated MyoII, in turn, increases cortical tension and promotes fusion pore formation. We propose that the protrusive and resisting forces from fusion partners put the fusogenic synapse under high mechanical tension, which helps to overcome energy barriers for membrane apposition and drives cell membrane fusion.

Ultrastructural Analysis of Drosophila Ovaries by Electron Microscopy

The Drosophila melanogaster ovary is a powerful, genetically tractable system through which one can elucidate the principles underlying cellular function and organogenesis in vivo. In order to understand the intricate process of oogenesis at the subcellular level, microscopic analysis with the highest possible resolution is required. In this chapter, we describe the preparation of ovaries for ultrastructural analysis using transmission electron microscopy and focused ion beam scanning electron microscopy. We discuss and provide protocols for chemical fixation of Drosophila ovaries that facilitate optimal imaging with particular attention paid to preserving and resolving mitochondrial membrane morphology and structure.

Actin-propelled invasive membrane protrusions promote fusogenic protein engagement during cell-cell fusion

Cell-cell fusion is critical for the conception, development, and physiology of multicellular organisms. Although cellular fusogenic proteins and the actin cytoskeleton are implicated in cell-cell fusion, it remains unclear whether and how they coordinate to promote plasma membrane fusion. We reconstituted a high-efficiency, inducible cell fusion culture system in the normally nonfusing Drosophila S2R+ cells. Both fusogenic proteins and actin cytoskeletal rearrangements were necessary for cell fusion, and in combination they were sufficient to impart fusion competence. Localized actin polymerization triggered by specific cell-cell or cell-matrix adhesion molecules propelled invasive cell membrane protrusions, which in turn promoted fusogenic protein engagement and plasma membrane fusion. This de novo cell fusion culture system reveals a general role for actin-propelled invasive membrane protrusions in driving fusogenic protein engagement during cell-cell fusion.

Drosophila and mammalian models uncover a role for the myoblast fusion gene TANC1 in rhabdomyosarcoma

Rhabdomyosarcoma (RMS) is a malignancy of muscle myoblasts, which fail to exit the cell cycle, resist terminal differentiation, and are blocked from fusing into syncytial skeletal muscle. In some patients, RMS is caused by a translocation that generates the fusion oncoprotein PAX-FOXO1, but the underlying RMS pathogenetic mechanisms that impede differentiation and promote neoplastic transformation remain unclear. Using a Drosophila model of PAX-FOXO1-mediated transformation, we show here that mutation in the myoblast fusion gene rolling pebbles (rols) dominantly suppresses PAX-FOXO1 lethality. Further analysis indicated that PAX-FOXO1 expression caused upregulation of rols, which suggests that Rols acts downstream of PAX-FOXO1. In mammalian myoblasts, gene silencing of Tanc1, an ortholog of rols, revealed that it is essential for myoblast fusion, but is dispensable for terminal differentiation. Misexpression of PAX-FOXO1 in myoblasts upregulated Tanc1 and blocked differentiation, whereas subsequent reduction of Tanc1 expression to native levels by RNAi restored both fusion and differentiation. Furthermore, decreasing human TANC1 gene expression caused RMS cancer cells to lose their neoplastic state, undergo fusion, and form differentiated syncytial muscle. Taken together, these findings identify misregulated myoblast fusion caused by ectopic TANC1 expression as a RMS neoplasia mechanism and suggest fusion molecules as candidates for targeted RMS therapy.

Asymmetric Mbc, active Rac1 and F-actin foci in the fusion-competent myoblasts during myoblast fusion in Drosophila

Myoblast fusion is an intricate process that is initiated by cell recognition and adhesion, and culminates in cell membrane breakdown and formation of multinucleate syncytia. In the Drosophila embryo, this process occurs asymmetrically between founder cells that pattern the musculature and fusion-competent myoblasts (FCMs) that account for the bulk of the myoblasts. The present studies clarify and amplify current models of myoblast fusion in several important ways. We demonstrate that the non-conventional guanine nucleotide exchange factor (GEF) Mbc plays a fundamental role in the FCMs, where it functions to activate Rac1, but is not required in the founder cells for fusion. Mbc, active Rac1 and F-actin foci are highly enriched in the FCMs, where they localize to the Sns:Kirre junction. Furthermore, Mbc is crucial for the integrity of the F-actin foci and the FCM cytoskeleton, presumably via its activation of Rac1 in these cells. Finally, the local asymmetric distribution of these proteins at adhesion sites is reminiscent of invasive podosomes and, consistent with this model, they are enriched at sites of membrane deformation, where the FCM protrudes into the founder cell/myotube. These data are consistent with models promoting actin polymerization as the driving force for myoblast fusion.

Myoblast fusion in Drosophila

The body wall musculature of a Drosophila larva is composed of an intricate pattern of 30 segmentally repeated muscle fibers in each abdominal hemisegment. Each muscle fiber has unique spatial and behavioral characteristics that include its location, orientation, epidermal attachment, size and pattern of innervation. Many, if not all, of these properties are dictated by founder cells, which determine the muscle pattern and seed the fusion process. Myofibers are then derived from fusion between a specific founder cell and several fusion competent myoblasts (FCMs) fusing with as few as 3-5 FCMs in the small muscles on the most ventral side of the embryo and as many as 30 FCMs in the larger muscles on the dorsal side of the embryo. The focus of the present review is the formation of the larval muscles in the developing embryo, summarizing the major issues and players in this process. We have attempted to emphasize experimentally-validated details of the mechanism of myoblast fusion and distinguish these from the theoretically possible details that have not yet been confirmed experimentally. We also direct the interested reader to other recent reviews that discuss myoblast fusion in Drosophila, each with their own perspective on the process [1-4]. With apologies, we use gene nomenclature as specified by Flybase (http://flybase.org) but provide Table 1 with alternative names and references.