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.
