By Jung Soo Lee ‘12, thurj Staff
Think of Prometheus, the Greek Titan eternally condemned by Zeus for stealing fire from him and sharing it with the mortals. For his crime, he was punished by being chained to a rock while an eagle pecked away at his liver every day, only to have it grow back for months and years of continuous torture.
While the story of this cruel and unusual form of punishment has captivated the minds of many, it also presents an interesting and yet fundamental question of regenerative biology. How is it that as little as 25% of a liver can regenerate to form a fully sized liver? What is it about the liver that allows it to fix itself in such a drastic way?
These kinds of questions have led to the field of regenerative biology, which has gained a great deal of attention for its controversial yet groundbreaking findings in stem cell research. One of the areas of stem cell research that has attracted growing interest from the greater research community and public is nuclear reprogramming.
Dr. Konrad Hochedlinger, an assistant professor of Harvard Medical School and a principal investigator at the Massachusetts General Hospital, leads a stem cell lab that is doing some of the most groundbreaking and cutting edge research on reprogramming.
Nuclear reprogramming is actually just what it sounds like. Reprogramming in general refers to the concept of changing the wiring, or the global gene expression, of a cell in order to change its identity. In the case of stem cell research, reprogramming refers to the genetic manipulation of a differentiated cell – a skin fibroblast, a cardiomyocyte, or even a β cell of the pancreas – to turn it into a cell that resembles an embryonic stem (ES) cell. The product of successful reprogramming is referred to as an induced pluripotent (iPS) cell. In 2006, Yamanaka and Takahashi of Japan first reported the derivation of iPS cells from mouse fibroblast cells in their seminal paper. Just a year later, various groups had succeeded in generating human iPS cells. Since then, reprogramming has become one of the major, if not the main, focus of today’s stem cell research.
To the average person, this almost seems like a technique straight out of a sci-fi movie. By reversing the time on a differentiated cell, researchers have been able to generate iPS cells that exhibit two important characteristics of an embryonic stem cell: self-renewal, or the ability to indefinitely make more copies of itself, and pluripotency, or the capacity to differentiate into more defined cell types.
For Dr. Hochedlinger, this phenomenal differentiation potential of embryonic stem cells and iPS cells drew him to stem cell research. He found it “fascinating that you can capture in a petri dish undifferentiated cells that can be coaxed into all cell types of the body.”
It is this much talked about concept of pluripotency and iPS cells that has gained the interest of many medical researchers who believe that the technique may allow the development of novel regenerative therapies and treatments for degenerative diseases of the human body.
Take, for instance, Parkinson’s disease, in which degeneration of midbrain dopaminergic neurons leads to severe loss of motor skills along with many other debilitating symptoms. In the near future, scientists may be able to take the patient’s skin cells from a simple biopsy and reprogram them into iPS cells. Those iPS cells can then be directed to differentiate into dopaminergic neurons which can then be transplanted back into the patient. Not only would this kind of therapy replace cells that are lost during disease, but it would also bypass the problem of immune rejection that always complicates transplant procedures because the replacement dopaminergic neurons would be generated from the patient’s own skin cells.
Although reprogramming may seem like the perfect solution for curing various degenerative diseases, there are still many obstacles to overcome in the original reprogramming method described by Yamanaka and Takahashi before they can be used to treat diseases in humans.
First of all, the efficiency of reprogramming remains very low and the procedure is quite time-consuming. In order for the procedure to be useful for medical purposes, it must yield considerable numbers of iPS cells for researchers to manipulate in the laboratory. As Dr. Hochedlinger describes, a possible strategy for improving efficiency is to “pick different cell types from ones used today [skin cells, fibroblasts]. We’ve shown in the blood system that if you start with immature cells, cells are more efficiently converted to iPS cells.” This approach makes intuitive sense, since less differentiated and “younger” cell types are closer in identity to embryonic stem cells. These “immature” cells would have less distance to travel before they are reprogrammed into iPS cells.
Another way to prove efficiency is to “inactivate pathways for inducing senescence in cells,” as Dr. Hochedlinger explains. Senescence refers to the point at which cells have aged so much that they stop dividing. Because the cell types used in reprogramming are often older, adult cells and the process takes many days to complete, allowing cells to reach an immortal state to allow them to avoid aging and senescence prior to reprogramming may improve the efficiency of the process.
Even if some of these barriers to use of iPS cells in therapies are overcome, it may be awhile before they can help treat numerous degenerative diseases. It is still not known “how to make a variety of true cell types from pluripotent cells,” says Dr. Hochedlinger. “We know how to make cardiomyocytes and neurons, but that’s about it. Another problem is how to engraft cells. Once you’ve made neurons, you have to know how to graft them to make them functional.” Yet another issue lies in the nature of iPS cells specifically. “We don’t fully understand if iPS cells are exactly equivalent to ES cells – if they do everything ES cells do.”
Despite the seemingly endless number of issues with reprogramming that need to be addressed, researchers are currently trying to solve all of these problems in order to perfect the promising technique.
Dr. Hochedlinger’s lab, for example, seeks to study not the end result of iPS cells, but the actual process of reprogramming. “Reprogramming takes about 10 days to 2 weeks in culture. We have no good understanding of what’s going on in those 2 weeks,” says Dr. Hochedlinger. “We’ve isolated cell surface markers that are downregulated and upregulated in cells that eventually become iPS cells. These markers allow us to pull out cells [to study them] at different time points when they’re not yet iPS cells.” The Hochedlinger lab is also studying the identity of iPS cells. “We’re trying to understand if embryonic stem cells are equivalent to iPS cells,” explains Dr. Hochedlinger. “We’ve developed a system in mice, so we can compare genetically identical ES and iPS cells. We’re using genome wide technologies to really look at every nucleotide in the genome to figure out what the difference is between the two cell types.”
With the many stem cell labs working to further refine the process of nuclear reprogramming, medical therapies using reprogrammed cells may not be out of the question. The original studies of Yamanaka and Takahashi that led to the derivation of the first iPS cells have led other researchers to develop different reprogramming methods. For example, direct lineage reprogramming involves the reprogramming of one adult cell type directly into another cell type without passing through an iPS cell stage.
Although at this point, it is difficult to assess how realistic the expectations of using reprogramming technology for disease treatments are, iPS cells are already making great contributions to researchers’ understanding of diseases. Deriving iPS cells from diseased cell types give researchers the ability to study diseases in a petri dish as they develop and unfold. By using iPS cells as human disease models, researchers are coming closer to developing new treatments for many incurable disorders.