Researchers have identified a protein that is critical for keeping adult cells from turning back into induced pluripotent stem (iPS) cells. In cells that lack this protein, the transcription factor cocktail that is used to turn cells into iPS cells worked basically without fail; the efficiency of generating such cells skyrocketed from 1 percent of cells to more than 90 percent.

Pluripotency is a double-edged sword. It is necessary for getting a whole organism out of a single cell. But in order to actually get anywhere with the work of making an organism, cells quickly need to give up their potential and settle down into one role. And stick with it.

As a result, during development, Jacob Hanna told BioWorld Today, “the stem cell state is not retained – it is very transient.”

Pluripotency lasts for only a few days, and by the time the embryo implants into the uterine wall, the party’s over. “This brake comes on, and it stays on, in every cell in our body” – including the adult cells that are reprogrammed in the generation of iPS cells. Given this state of affairs, the fact that induced pluripotent stem cells (iPSCs) exist at all – that it is possible to reprogram an adult cell into an embryonic stem cell-like state by the addition of a mere four transcription factors – is astonishing.

And the reason why most cells do not turn into iPS cells even when reprogramming factors are added becomes obvious. Such transduction in the presence of inhibitory factors is essentially an attempt to push them into pluripotency without removing the obstacles in their way.

Hanna, who is at the Israeli Weizmann Institute, likened reprogramming under the influence of the repressor to “driving a car by pressing on the gas and the brake at the same time.”

In their experiments, which were published in the Sept 18, 2013, advance online edition of Nature, Hanna and his colleagues first identified Mbd3 as a possible master of repressing stem cell-like behavior through screening experiments. They found that when they depleted cells of Mbd3 before adding the usual cocktail of reprogramming factors, the efficiency of reprogramming skyrocketed to nearly 100 percent. Moreover, the cells reprogrammed more or less in lockstep, rather than at random times after the addition of the reprogramming factors.

Last week, Spanish researchers reported that they have managed to make iPS cells directly in vivo, rather than in cell culture. (See BioWorld Today, Sept. 12, 2013.)

Hanna said that blocking Mbd3 could also be done in vivo, so that the two methods could potentially be combined – although he was skeptical of the Spanish team’s assertions that the in vivo cells are closer to totipotency than those produced in culture, calling those assertions “unproven claims.”

Hanna said that the work could improve the chances for using iPS cells clinically directly. Even though such transplantation does not need numbers of cells that are out of reach with the current efficiencies. But Hanna noted that to make such cells true contenders for the clinic “we need methods that do not modify the genome.” The efficiency of such methods is currently far below the 1 percent that can be achieved by inserting the genes for transcription factors, and so increases makes it much more realistic to use such methods for generating iPS.

But the work may ultimately be more influential by allowing scientists to better understand iPS cells than by its effects on their ability to make the cells in the first place.

Hanna pointed out that because each adult cell transforms into an iPS cell according to its own timetable, and only 1 percent of them transform at all, it is currently a tall order to study such cells prospectively. Basically, the only way to do so is to follow 99 cells that don’t transform for every cell that does and yields data – a daunting enterprise no matter how much high-speed computing and high-throughput measurements are applied to it.

As a result, transformation has remained a black box of sorts – until now. The team’s discovery of Mbd3 means, Hanna said, “that we can finally study the mechanisms of transformation, because we are not dealing with only 1 percent of cells.”