The genetic approach to reprogramming, pioneered by Shinya Yamanaka who won the 2012 Nobel Prize in medicine for his work, allows human cells to be transformed back into an embryonic-like, pluripotent state. The promise is that a patient's own cells could be grown and used to fight disease and regenerate tissue after an injury.
This method involves taking a sample of a subject's cells, for example fibroblasts (or skin cells), and introducing a set of four genes into them via retrovirus.
"Typically it takes more than one to two months--to start from human skin fibroblasts or blood cells, to a well characterized, established induced pluripotent stem cell culture -- at least a couple of months," said Dr. Sheng Ding.
This becomes a game of numbers.
"When you put those genes and virus into the cells," said Ding, "then one out of thousands or ten thousands of cells actually would become --ultimately become--iPS cells."
Most of the cells in such a culture will not reprogram properly, but a small number will--and this small population is the one that the lab proceeds to grow over the course of many months, testing their viability and quality against that of pure embryonic stem cells.
"So in a way, this process is a process of selection --but this kind of selection is certainly not efficient. It takes so long, but --it's sort of like evolution--when you select cells with some sort of growth advantage, that again would come with safety concerns--perhaps, like cancer cells, they grow better, and that came out of the selection process."
But no clinic wants to transfuse potentially cancerous cells into a patient.
Ding, Senior Investigator at the Gladstone Institute of Cardiovascular Disease and a professor at University of California San Francisco, is one of the scientists aiming to solve this problem by generating stem cells using chemical methods.
Improving the reprogramming efficiency and accelerating the speed is just one aspect of using small molecule cocktails --rather than retroviruses--for reprogramming. A non-genetic means of cell manipulation, non-genetic approaches, can create safer stem cells.
A chemist by training, working in synthetic, organic chemistry, and making molecules, Ding found himself, like many, drawn to the stem cell field.
"How cells can develop into a whole organism--the development, the regeneration. These are well observed phenomena. Once I made a lot of molecules, I wondered what would be the best place to discover the use of those small molecules, and with a fascination with stem cell and regenerative biology --that's how I got started."
The challenge for regenerative medicine.
A long term vision of Ding's lab is to bring the stem cell revolution to the pharmacy and the home, the way many current medications are delivered.
"When people talk about regenerative medicine, stem cells, or cell-based therapy," he said, "you can imagine a cell-based therapy with our current understanding, like bone marrow transplants, the ever improving techniques that people developed, would be more and more useful."
But they're not convenient. "You have to find the right donor. But also --even when you have the right type of cells ... very often even in bone marrow transplants and other well accepted procedures, the recipient has to be conditioned --with chemotherapy and radiation-- to really create room for receiving the transplant itself. So that has risks."
Also, the transplantation will take place in a hospital --and not something that would be practiced at home.
"So the bottom line is, right now it's very hard to imagine you could ultimately go to a pharmacy to buy a vial of cells on the shelf to regenerate or repair any cells lost in disease or injury.
"So, an alternative approach to a cell based therapy is really about the conventional approach to drug development --small molecules--proven therapeutics, which we can actually take at home under a prescription."
Taking a pill that tells your body how to heal itself?
"In every organ of our bodies," said Ding, "we do have those preserved stem cells, tissue specific stem cells, regenerative cells, or even more differentiated cells--and those cells normally respond to a tissue's homeostasis needs --in many of our organs those stem cells would supply regularly cells to maintain homeostasis."
To keep everything balanced and maintain the 'status-quo', as it were. "Those cells will respond to disease and injury to repair and regenerate --although to a much less an extent than would be the case with other animals that can regrow lost limbs."
"We have those cells that can respond to signals," said Ding. "But we don't have the robust signal to regenerate."
"The notion is really about, can we actually develop the conventional small molecule or biologics that can be taken by a patient in a really convenient way --and those molecules can act perhaps in a tissue specific manner to stimulate our bodies' own stem cells to regenerate, to repair?"
"So that's more of a long term vision of why we actually do small molecules --discovery work in terms of finding molecules that can really control stem cell fate in terms of differentiation and reprogramming, stem cell growth."
So far Ding's lab has discovered over twenty different small molecules --more than half of which they've published already in the top peer-reviewed journals such as Cell Stem Cell and Nature Cell Biology.
"And many of those small molecules are widely used in many laboratories --in terms of either just making better iPS cells or using those molecules to study the mechanism.
"But certainly most of our patents are around making iPS cells or licensed to biotech companies in terms of further developing those as therapeutics or a more practical sort of use of those technologies."
Once Ding identifies a small molecule, then he researches its history, its pathway. "We really want to tell a good story --not just what it does but also why it does it. In what mechanisms--and once we know that, we publish the molecule and researchers would have access to our publications freely and use it.
"All of our published small molecules are commercially available from many vendors --chemical vendors-- and also biotech companies are moving forward on some of the molecules."
Ding sees one immediate advantage of the chemical approach to cell reprogramming, is in the business of disease modeling, and drug discovery. But he also has his eyes on the potential for tissue regeneration.
Like many other labs, he's invested in high-throughput screening technology to accelerate the process. "We have desktop automation instruments that really facilitate us to screen thousands or hundreds of thousands of compounds in parallel in a fast, economic way.
"By hand, typically is slower, but now we actually just do things in parallel and in terms of treating cells, adding small molecule drugs, and also analyzing the phenomenon those small molecules may create --it's all by automation. With algorithms recognizing specific cell phenotypes."
Small molecules also help to learn more about the underlying mechanisms of cell reprogramming.
"The better way to understand a phenomenon or process is--by perturbing it. When you perturb it, you see a difference that can provide you an entry point in terms of understanding the mechanism.
"In our case, we would have small molecules that can, say accelerate reprogramming and enhance reprogramming efficiency --the molecule can really substitute for those genes in terms of generating the reprogramming of the cells.
"The small molecule itself --not only does the molecule perturb the system, give you a difference, a change of the process that provides one entry point, but the molecule also provides a probe --a handle. It not only tells the cell how to change, the molecule has to physically interact with proteins inside the cells, to elucidate its function."
In that sense, he added, it's sort of like fishing. "You can use the molecule to fish out proteins that are responsible for the small molecule's activity in the process. So that fishing experiment --it's really a target identification experiment --will really tell you which proteins are responsible, are part of the underlying reprogramming process.
What's the next big breakthrough in the field?
"Certainly, in the past, the big breakthrough was assumed to be regeneration with iPS cells. That is why Yamanaka got the Nobel prize. But in the future, in my view, the next breakthrough would really be developing the actual drugs that stimulate our body's own cells to regenerate so that we don't have to transfuse a patient's own cells."
Finding the small molecules that can 'awaken' the body's own cells to regenerate more efficiently.
"So, if we can do that in a very controlled manner," said Ding, "I think that really will be the Holy Grail."
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