Macrophage Cell Fusion and Crosstalk with Myoblast Fusion

Cell-to-cell fusion plays a pivotal role in various developmental processes, tissue homeostasis, immune response and, possibly, cancer. One of the key challenges in characterizing these complex and relatively slow membrane fusion events is to uncouple actual fusion stage from the preceding differentiation processes, which prepare the cells for fusion. Here we have focused on two very important and well characterized examples of cell-to-cell fusion, macrophage fusion that leads to osteoclast or giant cells formation and myoblast fusion in muscle development and regeneration. Proteins that mediate fusion stages of these processes remain obscure. Application of RANKL to RAW macrophage-like cells commits the cells to fusion with most fusion events to take place 72-96 hours later. We blocked this robust fusion by applying a reversible hemifusion inhibitor lysophosphatidylcholine (LPC) at 72 hours post-RANKL, and removed LPC at 88 hours. This approach has allowed us to accumulate the ready-to-fuse macrophages and then observe cell fusion events that would normally develop within 16h to develop within 30-90 min. Synchronization of cell fusion using LPC block has also worked for myoblast fusion. Antibodies against annexin V inhibited both macrophage fusion and myoblast fusion suggesting similarities between protein machineries involved in these fusion processes. We also found that co-incubation of fusion-committed RAW cells and primary myoblasts promotes myotube formation and inhibits osteoclast formation suggesting an intriguing cross-talk between these cells in the context of muscle regeneration as well as in inflammation & bone biology.