Mechanisms of cell-cell fusion in development and disease

Our lab is interested in understanding the mechanisms of cell-cell fusion. Although cell-cell fusion occurs in several specialized cell types, it is critical for the conception, development and physiology of multicellular organisms. For example, sperm and egg fusion initiates zygotic development; fusion between muscle cells (known as myoblasts) leads to the formation of multinucleated, contractile muscle fibers during muscle development and regeneration; and fusion between macrophages results in multinucleated giant cells during immune response. Cell-cell fusion has also been implicated in the formation of bone and placenta, tumorigenesis and stem cell-mediated tissue repair. Despite the diversity of cell types that undergo fusion, all cell-cell fusion events involve cell recognition, adhesion, and membrane merger, suggesting that shared molecular mechanisms may be used.

With the long-term goal of revealing the general mechanisms underlying cell-cell fusion, we focus our investigations on myoblast fusion, an indispensible step during skeletal muscle development and stem cell-mediated muscle regeneration. We primarily use the model system Drosophila for our studies, since myoblast fusion in Drosophila is a highly conserved process, yet it is relatively simple and genetically tractable. Starting from a forward genetic screen, we first identified a collection of genes required for myoblast fusion in vivo, and subsequently placed these genes in a signaling cascade that transduces the fusion signal from the cell membrane to intracellular components. Interestingly, most of the �fusion genes� identified to date are linked to actin cytoskeleton remodeling, indicating an essential role for actin polymerization in cell membrane fusion.

Our subsequent studies using a multifaceted approach combining genetics, immunohistochemistry, live imaging and electron microscopy have led us to pinpoint an important function of the actin cytoskeleton in myoblast fusion. Contrary to the common belief that cell-cell fusion is a symmetrical process between two fusion partners, we show that myoblast fusion is mediated by a cell type-specific, F-actin-propelled podosome-like structure (PLS), which invades the apposing fusion partner with multiple protrusive fingers to promote fusion pore formation. We further demonstrate that the dynamics of actin polymerization and the proper assembly of actin filaments within the PLS are critical for its invasion.

Based on the insights obtained from our studies of myoblast fusion in Drosophila embryos, we have recently reconstituted high efficiency cell-cell fusion in cultured cells that otherwise do not fuse. This is achieved by co-expressing Drosophila adhesion molecules and a transmembrane fusogenic protein from C. elegans in cultured Drosophila S2R+ cells. We show that both fusogenic proteins and actin cytoskeletal rearrangements are necessary for cell fusion, and in combination they are 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 not only reveals a general role for actin-propelled invasive membrane protrusions in driving fusogenic protein engagement during cell-cell fusion, but also provides an exciting platform for detailed mechanistic analysis of the fusion process and for genome-wide screens of new components of the cell-cell fusion machinery. We hope that our future studies will continue to shed light on the fascinating biology of cell-cell fusion, and ultimately provide basis for optimizing stem cell-mediated tissue regeneration in genetic and acquired diseases.