Muscle cells originate from precursor cells known as myoblasts. Myoblasts fuse to form a single long, tubular cell called a myocyte (a muscle fiber). Muscle tissue is composed of large collections of these fibers. The fusion of myoblasts into muscle fibers takes place early in fetal development. With exercise and throughout a person’s life, the process is repeated to form new muscle mass and repair old or damaged muscle.
It takes many hours for cells to prepare for fusion, but the fusion process itself is very rapid. To study myoblast fusion, researchers first blocked the start of the fusion process with a chemical. Ordinarily, the mouse myoblasts the researchers worked with fuse at varied intervals. By blocking fusion, and then lifting the block, the researchers were able to synchronize fusion in a large number of cells, making the process easier to study.
Researchers in the Section on Membrane Biology, part of the Program in Physical Biology in the Division of Intramural Research, identified the two distinct stages of cell fusion and the essential proteins that facilitate these stages. In the first stage, two myoblasts meet, and proteins on cell surface membranes cause the membranes to meld. In the second stage, a pore opens between the cells and their contents merge. This second step is guided by proteins inside the cells. The researchers identified the protein dynamin, found inside the cell, as essential to the second stage of the cell fusion process.
The findings have implications for understanding how to repair and rehabilitate muscle tissue. The findings may also contribute to understanding other processes that involve cell fusion, such as when a sperm fertilizes an egg, when viruses infect cells, or when specialized cells called osteoclasts dissolve and assimilate bone tissue to repair and maintain bones (PMID: 23277424).