In mammals the one thing that never happens under normal circumstances is for a cell to dedifferentiate, that is, revert back to a more primitive type. Indeed, the only exception to this rule is cancer cells, which can become less differentiated than the tissue in which they first arise. Unfortunately, some cancer cells can also continue to divide endlessly, displaying an immortality similar to that of pluripotent cells.
Until recently, the only way to turn back the developmental clock of a normal adult cell was through elaborate manipulations to trick it into behaving like an embryonic cell, a process termed cellular reprogramming. The oldest approach to achieving reprogramming is somatic cell nuclear transfer, or “cloning,” which involves injecting the genetic material from an adult cell into an egg cell whose own DNA has been removed. This DNA-egg hybrid then develops into an early-stage embryo from which pluripotent stem cells can be extracted.
Since the cloning of Dolly the sheep was revealed in 1997 and the first isolation of human embryonic stem cells in 1998, nuclear transfer has received considerable attention as a possible means of producing custom-tailored pluripotent stem cells to replace any tissue damaged through injury or disease. Poorly understood factors within the egg do seem to genuinely rejuvenate the genetic material of the adult donor cell—even telomeres, the caps protecting the ends of chromosomes that wear away with age, are restored to a youthful state. Yet despite progress with animals, attempts to produce human embryonic stem cells through cloning have remained unsuccessful.
Yamanaka and his group went around this impasse by taking a novel approach to turning adult cells directly into pluripotent cells without the use of eggs or embryos. Instead of introducing adult genetic material into an egg, they reasoned that introducing the genes normally active only in embryos into an adult cell might be sufficient to reprogram that cell into an embryolike state. Their first feat was to identify a cocktail of two dozen different genes that are turned on in pluripotent cells but silent in adult cells. When introduced into skin cells using retroviruses as delivery vehicles, these genes then almost magically reprogrammed the identity of the skin cells into that of pluripotent cells. With further experiments, Yamanaka then found that only four genes—Oct4, Sox2, Klf4 and c-Myc—were actually necessary to produce iPSCs.
As soon as several independent laboratories, including mine, successfully reproduced the results, this magic trick became a biological fact. By now about a dozen different adult cell types from a total of four different species (mouse, human, rat and monkey) have been reprogrammed into iPSCs, and certainly more will follow. The discovery of iPSCs is so thrilling to stem cell researchers because they can circumvent the technical complexities of cloning and avoid most of the ethical and legal constraints associated with human embryo research. This new pluripotent cell type is not without its own problems, however. Quality control and safety are the main focus of iPSC research right now, as scientists work to establish what these cells really are and what they are capable of doing.
Although iPSC colonies may look like embryonic stem cells under a microscope and may display the molecular markers associated with pluripotent cells, the unequivocal proof of their pluripotency comes from functional testing—can the cells do all the things a pluripotent cell, by definition, can do? Even embryo cell colonies can contain some dud cells that do not display the pluripotency of a true embryonic stem cell, and scientists have developed a few routine tests to gauge a cell’s pluripotency. With increasing stringency, they are: the ability of stem cells to produce a wide variety of body cell types in a petri dish when exposed to the appropriate developmental cues; the ability of stem cells to produce a teratoma (a type of tumor containing cells from all embryonic tissue lineages) when injected under the skin of a mouse; and the capacity, when injected into an early-stage mouse embryo, to contribute to the development of all tissue lineages, including germ cells, in the resulting newborn mouse.