|
||||||||
|
DIABETES RESEARCH New Source of Insulin-producing Cells FoundAs recent research on adult stem cells has shown, sometimes you can teach an old cell new tricks. Susan Bonner-Weir, HMS associate professor of medicine at the Joslin Diabetes Center, and her group have discovered a way to turn pancreatic tissue into structures that resemble the precious insulin-producing islets of Langerhans. The method, published in the July 4 PNAS, uses ductal tissue that is normally discarded by researchers after isolating islets from the pancreas of a human cadaver. This resourceful approach may help supplement the scarce supply of islets available for transplantation into diabetic patients.
Pancreatic ductal tissue grown in culture transforms into a cultivated human islet bud (CHIB) complete with insulin-producing beta cells (dark red). In June a group of researchers from the University of Alberta announced they had performed successful islet transplants that allowed eight patients with severe diabetes to survive without insulin injections for over a year and counting. The announcement caused a stir in the emerging field of islet transplant research and, in response, 10 sites worldwide—including the Juvenile Diabetes Foundation Center for Islet Transplantation at HMS—were recently named to take part in a study of the new protocol. But there is still the problem of how to obtain islets, which are in scarce supply and must be taken from cadavers. AlchemyBonner-Weir's fortuitous discovery began when her colleague Jack O'Neil complained that non-islet cells were growing in flasks of isolated human islets. Islets make up only about one to two percent of the pancreas, and it is difficult to isolate them from other tissues. Bonner-Weir identified these adherent cells in O'Neil's isolations as duct cells, and asked if she could have them.With the help of Monica Taneja, now a second-year medical student at the University of Pittsburgh, Bonner-Weir grew the ductal cells in a monolayer and then overlaid them with Matrigel, an extracellular matrix rich in growth factors. She hoped to see some of the ductal cells differentiate into islets, but the actual result was "one of those eureka moments," she says. "After a week we found that there were these isletlike structures that had lots of insulin in them." The new structures, dubbed cultivated human islet buds (CHIBs), grew as islet cell clusters protruding from 3-D ductal cysts. She has replicated this process using tissue from eight pancreases. The insulin production of the tissue increased 10- to 15-fold in less than a month in culture, and the CHIBs have secreted insulin in response to glucose.
Clockwise from lower left are members of Susan Bonner-Weir's lab: Arun Sharma, Michael Tenofsky, Ki-Ho Song, Jack O'Neil, Krystyna Tartarkiewicz, and Bonner-Weir, who embarked on a recycling project with pancreatic tissue that may aid people with diabetes. Not pictured are Monica Taneja and Gordon Weir. The SourceAccording to Bonner-Weir, it makes sense that ductal cells would differentiate into other pancreatic cells under the right conditions, given what is known about the embryology of the pancreas. The organ begins as an outcropping of the primitive gut, from which epithelial cells branch into a tree of ducts before differentiating into other specialized cells. "It is the duct cells, embryologically, that give rise to the other phenotypes," she says. Previous research on rats showed that adult duct cells can reenact this embryological development. "Our hypothesis based on the rat data is that as mature duct cells replicate, they lose some of their ductal specialization and regain their multipotency so that they then can respond to a second set of stimuli and can differentiate into islets," she says. The duct cells can act as a reservoir of new tissue, essentially being the functional stem cells of the pancreas.
Portrait of a CHIB: A cluster of islet tissue is shown emerging from a ductal cyst like the stone on a signet ring. By expanding these more abundant ductal cells, the method avoids some of the pitfalls that researchers have encountered trying to expand islets themselves. Human islet cells, even fetal ones, replicate at a very low rate, so expanding them is a difficult task. Some researchers have been able to increase islets in culture by stimulating them with growth factors and extracellular matrix, but with increased replication the cells tend to lose their ability to produce insulin. Growth PotentialBonner-Weir calculates that their current method would yield only about 30,000 to 35,000 islets from one pancreas, well below the half million or so necessary for a transplant. However, the approach has not yet been optimized, and she believes there is a lot of room to improve the output. The CHIBs her team cultivated developed without specific manipulation beyond providing the extracellular matrix; the team's next goal is to identify specific factors that could help coax the islet differentiation. Currently, the buds have equal numbers of insulin-producing beta cells and glucagon-producing cells, similar to a developing islet. But Bonner-Weir's team would like to direct the CHIBs to become more like mature islets, with a higher beta cell content.The Edmonton procedure re-quires pancreases from two cadavers for every patient, making it impractical on a large scale unless creative sources for additional islets are found. Researchers at Joslin have looked at using pig islets for transplants, which also shows promise although there are potential problems of xenograft rejection. Bonner-Weir's method is an attractive way to supplement the islet supply because it avoids the difficulties of xenotransplantation as well as the thorny issues of using embryonic stem cells. —Courtney Humphries |
