While attractive, the idea of reprogramming adult cells to produce embryonic-like stem cells has not been implemented with much satisfaction. Reprogramming has remained frustratingly slow and inefficient, and it results in stem cells that have limited utility. But now a key impediment to reprogramming has been recognized. Its removal, according to an investigation described today in Nature ("Deterministic direct reprogramming of somatic cells to pluripotency"), not only shortened reprogramming time by several orders of magnitude, it also improved the efficiency of the process—all the treated cells attained a stem-cell-like state, and they all did so, conveniently, at the same rate.
Adult stem cells may be reprogrammed by inserting four genes into their DNA, a process that yields induced pluripotent stem cells (iPSCs). The process, however, is fraught with difficulty. It can take up to four weeks, the timing is not coordinated among the cells, and one percent or less of the treated cells actually end up becoming stem cells. Success rates can be even lower—around a tenth of a percent—if stem cells are to be used in patients. In such cases, viral gene insertion techniques must be shunned for safety reasons.
The question of efficiency was taken up by researchers at the Weizmann Institute of Science who were already investigating the natural pathways of embryonic development. In particular, researchers in the laboratory of Yaqub Hanna, M.D., Ph.D., asked: What is the main obstacle—or obstacles—preventing successful reprogramming in the majority of cells?
In his postdoctoral research, Dr. Hanna had employed mathematical models to show that a single obstacle was responsible. The identity of the obstacle, however, remained unclear. Then, scientists in Dr. Hanna’s laboratory looked at a certain protein, Mbd3. This protein, a core member of the Mbd3/NuRD (nucleosome remodelling and deacetylation) repressor complex, had caught their attention because it is expressed in every cell in the body, at every stage of development.
Such a protein is quite rare. In general, most types of proteins are produced in specific cells, at specific times, for specific functions. The team found that there is one exception to the rule of universal expression of this protein—the first three days after conception. These are exactly the three days in which the fertilized egg begins dividing, and the nascent embryo is a growing ball of pluripotent stem cells that will eventually supply all the cell types in the body. Starting on the fourth day, differentiation begins and the cells already start to lose their pluripotent status. And that is just when the Mbd3 proteins first appear.
The researchers showed that removing Mbd3 from the adult cells can improve efficiency and speed the process by several orders of magnitude. Efficiency approached 100% from mouse and human cells, and the time needed to produce the stem cells was shortened from four weeks to eight days. As an added bonus, since the cells all underwent the reprogramming at the same rate, the scientists will now be able, for the first time, to actually follow the process step by step and reveal its mechanisms of operation.
Recalling how his team’s discovery was based on research into embryonic development, Dr. Hanna said, "Scientists investigating reprogramming can benefit from a deeper understanding of how embryonic stem cells are produced in nature. After all, nature still makes them best, in the most efficient manner."
Dr. Hanna’s laboratory is conducting several investigations into iPSCs. These include deciphering the mechanisms of epigenetic reprogramming and induction of pluripotency in somatic cells, including fibroblasts and lymphocytes, as well as the development of iPSC-based experimental systems for in vitro modeling of human disease and development.