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BIOLOGICAL CHEMISTRY
Protein Identified As Marker for Stem Cell FusionIn the corporate world, a company wishing to grow in an entirely new direction might initiate a merger with another company that already has the knowledge and expertise required. In the biological world, it turns out, a similar strategy is adopted by cells.
ABCB5, a cell surface protein cloned by Markus Frank and colleagues, may be a marker for stem cell fusion. The protein is expressed in most progenitors found in human epidermal melanocytes, and antibodies to ABCB5 promote fusion of these cells. It has been demonstrated recently that when exogenous stem cells, particularly bone marrow cells, are introduced into mice, they actually merge with host cells to form polyploid hybrids. These findings suggest that stem cells, once thought to be inherently pluripotent, may derive at least some of their versatility from an ability to fuse with cells that are already differentiated. But exactly how such cellular mergers are handled is unknown. In a paper in the Nov. 21 Journal of Biological Chemistry, instructor in medicine Markus Frank and associate professor of medicine Mohamed Sayegh, both at Brigham and Women's Hospital, identify a protein that may not only be a marker for cells that can fuse, but also play a role in the fusion process itself. ABCs of Cell FusionFrank has been working on a family of cell surface glycoproteins called ABC transporters. These proteins have been the focus of cancer biologists because they very efficiently pump certain chemicals out of cells and by doing so can confer drug resistance to tumors. About three years ago, Frank noticed a region in the human genome homologous to the then known ABCs. He cloned the gene, which coded for a transporter dubbed ABCB5, and found it was predominantly expressed in the skin, a tissue known for renewing itself and serving as a lucrative source of pluripotent stem cells. At about the same time it was reported by others that two members of the transporter family, ABCB1 and ABCG2, are expressed in stem and progenitor cells, which made Frank wonder about the role of ABCs in stem cell biology. "Though ABCs are found in almost every living organism," he explained, "their functional relevance is still pretty unclear."
The cells had sometimes two, three, or even four times as much DNA as a normal diploid cell, suggesting that they were products of cell fusion. "This was very exciting," said Frank, "because it suggested to us the possibility of cell fusion in an endogenous progenitor subset, which is different from the finding that exogenous cells could fuse with endogenous cells."
Frank and colleagues found that ABCB5 is expressed in bi- and multi-nucleated cells that can give rise to normal diploid cells containing a single nucleus. The team includes (l to r) Peter Lapchak, first author Natasha Frank (sitting), Markus Frank, Shona Pendse (sitting), Armen Margdryan, and Debbie Shlian. Inset is associate professor of medicine Mohamed Sayegh. (Photo by Steve Gilbert) To be certain that these polyploid cells really arose by fusion, Frank and colleagues mixed two populations of HEM cells that had been labeled with different colored carbocyanine dyes, ones that cannot diffuse from one cell to another. When they examined the mixture an hour later, they found multinucleated cells that were dual colored, indicating that the cells had merged. Furthermore, when a monoclonal antibody raised against ABCB5 was included in the incubation, the numbers of dual-colored, multinucleated cells almost doubled, indicating that blocking the transporter promotes the cellular fusion. Fusion then FissionIt has been shown that fusion of exogenous bone marrow cells with host hepatocytes may be the primary means by which the former repopulate a diseased liver with healthy cells. So the fusion hybrids may be capable of yielding normal diploid progeny. Frank and colleagues found evidence to support this theory. The dual-colored, multinucleated cells were found to attach to the culture dish after about 24 hours, usually a sign that cells are capable of differentiating. To test this, the authors examined the cells after five days, when they found cell clusters containing dual-colored mononucleated cells. Because these could arise only after fusion, the results indicate that multinucleated cells can produce diploid daughter cells.How ABCB5 might contribute to fusion is not yet clear, but clues may come from its structure, which is unique among the transport family. All of the members have either one or two cytoplasmic ATP-binding sites, but ABCB5 is alone in having one of its two ATP sites on the outside of the cell membrane. "We don't know if this is related to fusion," commented Frank, "but it sets the transporter apart from similar molecules and suggests unique roles not served by others. Alternatively, the role of ABCB5 may be related to maintenance of plasma membrane potential, which has been shown to be a key factor in progenitor cell fusion. When the researchers blocked the transporter with the monoclonal antibody they found that progenitor-like cells were significantly depolarized. "The work is stimulating," according to Michael Gottesman, chief of the Laboratory of Cell Biology at the National Cancer Institute, who studies the transporter family. "I would be surprised if [ABCB5] didn't have a transport function, but this does not rule out a function in cell fusion," he said, cautioning that the work needs to be confirmed by more definitive genetic analyses. Frank and colleagues are currently pursuing this line of investigation. --Tom Fagan |
