Making Stem Cells Without Adding DNA
Adding Just Four Proteins can Turn a Cell into an Embryonic Stem Cell
The stem cell field moves fast. A few years ago, scientists learned that they could add four genes to a cell and turn it into something very much like an embryonic stem (ES) cell. These transformed cells are called induced pluripotent stem (iPS) cells to distinguish them from true ES cells.
Now, a new study shows that scientists can skip the genes and just add four proteins. This is huge because it greatly reduces the risk that the newly minted iPS cell will turn cancerous.
This means we are one step closer to using iPS cells to cure incurable illnesses like Parkinson's. And one step closer to repairing irreparable injuries like those that involve the spinal cord.
Of course there is still a long way to go. The studies have so far been done only in mice. They'll need to be repeated in humans to make sure that the whole process works in people too.
Also, this research is still at a very early stage and so the process has not been optimized. The scientists only managed to transform three out of 50,000 cells into iPS cells. They will need to increase the efficiency of the process if these cells are to be used medically.
But if scientists can get an efficient process going in human cells, then another hurdle will be overcome in using iPS cells to treat disease. And in improving human life without destroying an embryo.
Genes Have the Instructions for Making Proteins
Back in 2007, scientists were able to turn skin cells into iPS cells by adding four genes. But the genes didn't do the actual magic. Instead, it was the proteins that came from the genes' instructions that transformed the cells.
Each gene has the instructions for making a specific protein. And that specific protein goes on to do a specific job in the cell.
For example, the HBB gene has the instructions for making hemoglobin. And hemoglobin's job is to carry oxygen in our blood.
The four genes that turned cells into iPS cells were Oct4, Sox2, Klf4, and c-Myc. These genes have the instructions for making proteins with the same names. It is these four proteins and not the genes that did the actual work of turning a skin cell into an ES cell.
So it makes sense that just adding the proteins themselves should be able to transform the cells. But why would any scientist want to do this if genes work? Because adding a protein is safer.
Why Adding Proteins is Safer than Adding Genes
At first blush, it might seem odd that genes are more or less safe to add to a cell than the proteins they produce. But genes are more dangerous.
When a scientist adds a gene to a cell, it can sometimes turn on other genes. And those other genes can sometimes cause the cell to turn cancerous.
Genes are stored in
chromosomes like these.
A cell's genes are stored in long DNA structures called chromosomes. When a gene is added, it sticks itself into one or more of these chromosomes. If the added gene lands near another gene, it can affect how that nearby gene works.
The nearby gene might now cause the cell to grow uncontrollably or make the cell refuse to die. In other words, the added gene might cause the nearby gene to turn the cell into a cancer cell.
The second problem has to do with how long a protein is around. In this case, scientists want the proteins to be there just long enough to do their job. This is because these proteins can sometimes cause cancer by themselves if they are around too long.
Since most proteins wear out pretty quickly, this shouldn't be a problem. Unless the protein's gene is there to replace worn out copies of the protein. When this happens, working protein can stay around for a very long time.
This is exactly what happens when genes are added to a cell. The added gene often just keeps making protein long after the protein is still needed.
But if scientists add just the protein, then when it wears out, it won't be replaced. So adding just the proteins to the cell is safer. Unfortunately, adding a protein is not easy.
Getting a Protein into a Cell
Most methods for getting big molecules into a cell are harsh. This is OK with genes because they are made of sturdy DNA. They can be dissolved in harsh solvents, dried, heated, and abused in lots of other ways and still work fine. The same is not true of proteins.
Proteins are much more fragile and stop working under conditions where genes are still working fine. So scientists had to come up with gentle ways to trick the cell into taking up the protein.
In this study, the proteins could get in cells because scientists added a positively charged tail to the proteins. Cells take up these positive tails through special proteins on their outsides called receptors. The attached protein goes along for the ride.
All four proteins work well despite having the attached tail. And the cells took these proteins in just fine. But once in the cell, these proteins could not change the cell into an iPS cell. Not without a little help from a chemical called valproic acid or VPA.
VPA Helps the Proteins do their Jobs
The four proteins turn cells into iPS cells by reprogramming them. Basically they cause a whole set of genes to be turned off and a whole new set to be turned on. They do this by changing how the chromosomes are wound up.
Chromosomes are made of more than the DNA that make up genes -- they contain proteins called histones too. The DNA in chromosomes is wound around these histones. These DNA+histones then wind around each other over and over until the whole thing looks like a tangled mess. But it is actually quite organized.
Some parts of the DNA are near the surface of this tangle. Other parts are buried deep. Cells can read the genes near the surface and have trouble or cannot read the buried genes.
What the four proteins do is rearrange this tangle in such a way that the genes that an iPS cell uses rise to the top. And many of the original genes the skin cell used get buried. The end result is an iPS cell.
This is not an easy job for these proteins! Each cell has around 6 feet of DNA tangled in a certain way. And these tiny proteins need to reorganize all of that DNA into a way that the cell can become an iPS cell.
On their own, these proteins can get partway there. Many of the treated cells in this study had some but not all of the qualities of the iPS cell. This is where VPA can help.
VPA sort of loosens the tangle making rearranging it much easier. When the scientists added VPA, the four tailed proteins were able to transform a few of the cells all the way to iPS cells.
The next steps are to optimize the conditions to get more iPS cells per protein treatment. The scientists might try adding different amounts of protein at varying times. Or they might try other protein helpers like VPA.
The ultimate goal would be to try to find some combination of chemicals that can turn a cell into an iPS cell without added genes or proteins. Then the process could be scaled up easily so that enough iPS cells could be made to