The stem cell frontier, 2008, on display, By Steve Clark
Cut a flat worm in two, the tail will grow a new head and the head a new tail. Cut it right down the middle, it will grow a mirror image. How does it know what to grow?
Flies can re-grow damaged tissues. Small fish can regenerate heart muscle. Why can’t humans?
In a developing infant, how do human embryonic stem cells know to grow into heart, muscle, liver, neurological and other cell types at the proper time and in the proper place?
These questions are at the center of science’s brave-new-world of stem cell biology and were the topic of the recent 3rd Annual Stem Cell Symposium. The science was deep and detailed, and enormously enthralling. It was an intellectual playground of exciting ideas and fabulous potential.
The overriding lesson from the conference is that the mechanism by which stem cells regulate how and when they replenish themselves vs develop into different tissues is conserved in species as divergent as worms, flies, fish and mammals. This is fascinating for developmental biologists, but it also has a profound practical impact for eventually using stem cells to treat human disease. Let me explain how.
A primer on stem cell science
Consider for a moment what needs to be accomplished for an immature stem cell to differentiate into, say a beating heart cell. First, there needs to be a stimulus that initiates this program, telling the stem cell to specifically move along the cardiac muscle developmental pathway. The stem cell must then begin expressing heart cell genes while repressing the expression of all other genes that could cause it to become liver, blood, kidney and all other cell types. Quite a tall order!
Once the stem cell develops into a mature beating heart cell, it remains that type of cell. Mature cells, like zebras, cannot change their stripes. We never see a heart cell become a skin cell and vice versa. Cellular development is unidirectional and this has been one of the central tenants of developmental biology.
Then along comes Scottish scientist, Ian Wilmut, who did an experiment in the mid-1990s that no self-respecting developmental biologist would attempt since we all “knew” that cell development only moved in one direction.
What Wilmut did was to remove the nucleus from an egg cell and replace it with the nucleus from a fully mature cell taken from a different animal. Keep in mind that this donor nucleus had already been directed to express only those genes of the tissue it was taken from and to repress the expression of genes from all other tissues.
Wilmut then transferred this engineered egg into the womb of a pseudo-pregnant sheep, where the engineered egg should have died. Instead, a sheep was born that was a genetic twin of the nucleus donor sheep and the world was introduced to the first cloned animal, Dolly.
This is the type of research result that causes a scientific paradigm shift. For the first time, we realized that the genetic program of a fully developed adult cell, when placed in the proper environment, can be reprogrammed to relinquish its adult cell properties and return to its undifferentiated stem cell state, capable of developing into a fully grown sheep.
Around the same time that Dolly was born, University of Wisconsin-Madison scientist, Jamie Thomson, published his seminal studies demonstrating the ability to grow monkey and human embryonic stem cells (or ESCs). These, of course, are the immature cells derived from five day old embryos that are able to develop into all tissues of the adult body. The way that ESCs are harvested kills these embryos making ESC research highly controversial. It would be great to be able to obtain such embryonic stem cells without having to destroy a functional human embryo.
Fast forward ten years to the conference where Professor Thomson gave an update on his recent report that he can reprogram adult cells to become stem cells without having to transplant cell nuclei. Looking at recent research from different labs, he noted that only a few regulatory genes are needed to maintain cells in their nascent developmental stage. As the research presented at the conference illustrated, these regulatory genes work across different species, so this mechanism is highly conserved in biology.
Thomson used routine gene transfer technology to induce expression of three different regulatory genes in the cells of mature fibroblasts and, amazingly, the mature cells were re-programmed to become stem cells! What Wilmut was able to do by transferring a cell nucleus to an enucleated egg can now be done in a petri dish and without the egg cell.
At the conference, Thompson explained that these “induced pluripotent cells” or iPCs seem to behave exactly like ESCs. Think about the implication of this observation: it means that mature cells from an adult can be re-programmed back to the stem cell state where they are able to generate anew, all tissues of the human body.
What next for stem cells?
UW-Madison stem cell researcher, Clive Svendson, moderator of the conference, believes that the next major advance will be the ability to develop iPCs by simply changing the environment in which adult cells are grown in the lab, which could be accomplished in about a year. This means that we would not have to insert several genes into a cell’s DNA, which has significant risks and is not a trivial procedure. Thus, it soon may be very easy to take cells from your skin, put them into a defined tissue culture environment and develop stem cells that contain your precise genetic makeup. No embryos would be destroyed and no clones would be created in the process, mitigating most of the ethical concerns.
Svendson opined that this could lead to a big boost in the tissue banking business as people store tissues when they are young for making stem cells if they should need them later. This would be necessary because, as Thomson explained, chronologically young cells are more efficient at being reprogrammed than cells from older animals. One can envision that it could become routine at birth to store placental tissue, the youngest tissue readily available that is genetically identical to the newborn baby.
As exciting as the science was at the conference, there remain some problems to deal with before these stem cells are used in the clinic. First, as with ESCs, undifferentiated iPCs form tumors called teratomas. Therefore, we need to develop a fail-safe way to completely separate or incapacitate contaminating stem cells from the functional tissues grown from them before we put them into patients. According to an article I posted here earlier, the FDA recently convened a meeting to grapple with this problem in anticipation that clinical trials will begin in the near future.
Next, even if we can use stem cells to regenerate damaged tissues, we still need to continue research into the causes of degenerative diseases because simply replacing the dying cells without dealing with what causes them to die may only be a short term fix.
Potential ethical issues may still arise
Finally, and potentially an explosive issue, there remains an ethical question regarding iPCs that no one seems to have addressed. When a mature cell is reprogrammed, how far back does it go? Do iPCs only have the potential to develop into different body tissues, or can an iPC, if given the chance, form an embryo? Of course, if the iPC cells are more like fertilized eggs than stem cells, then all bets are off--the ethical issues will arise again.
I asked both Svendson and Thompson about this and both admitted that this idea had not been tested. Svendson even owned up that no one in the field wants to test it. They do not want to know the answer because it could be very inconvenient.
My prediction
Carl Gulbrandsen, Director of the Wisconsin Alumni Foundation, shared with me that reading Thomson’s paper on reprogramming adult cells to derive embryonic iPCs made him “tingle”. Mr. Gulbrandsen does not seem to be the tingly type, but his response to Thomson’s results was not inappropriate—they are that amazing and significant.
As a scientist, I have learned to be cautious about making predictions. However, I venture one prediction here that I believe has a very good chance of being realized: Professor Jamie Thomson will, in the not-to-distant future, be awarded the Nobel prize for his outstanding work that has created a whole new field of stem cell biology and invigorated the practice of regenerative medicine. While I am at it, you can bet that he will share the prize with Ian Wilmut.
Any takers?
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This post was first published in-part by the Wisconsin Technology Network News
© 2008 Steven S. Clark, PhD. Disclaimer: The authors used their best efforts in collecting and preparing the information published herein. However, Steven S. Clark, nor other authors, do not assume, and hereby disclaim, any and all liability for any loss or damage caused by errors or omissions, whether such errors or omissions resulted from negligence, accident, or other causes.
Articles contained herein, are meant to be distributed freely to interested parties. However, any excerpts from any article must credit BioScience Biz.
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