Organ regeneration example from induced pluripotent stem cells. (Photo credit: Wikipedia)
Perhaps the biggest takeaway from last week's stem cell conference in Boston is the variety of approaches scientists now have at their disposal to study disease and tissue development, and to test drugs.
All through reprogramming. Turning one type of the human body's cells ... into any other cell type.
The most well known approach is the 'gene-therapy' approach of Shinya Yamanaka--introduce key factors into the cell via retro virus to push it into a more pluripotent state, a state from which the cells can be differentiated into neurons or heart cells.
But it's a complicated process, and one that faces many challenges before we see it used routinely in regenerative medicine.
Dr. Charles Murry, Professor of Pathology, Bioengineering and Medicine/Cardiology at University of Washington, discussed this at a special panel devoted to the evolution of iPS Cell Research.
"The challenges are, to get the cells to the proper stage of maturation," he said. Many of the most important diseases that stem cell researchers hope to treat, such as Alzheimer's, Parkinson's Disease, and ALS, really present in adulthood.
"They've probably been continuing the degenerative process [in the body] for some time, but I think we'll be able to model them better if we can get neurons or any diseased cell type of interest into an adult state."
They need to be put into their proper context, he added. "I think if we can work with colleagues in bioengineering, for example, to create a three-dimensional environment, that better recapitulates the cell's native environment in vivo --so that they could have the surrounding cells and connective tissues, things like that--we could make little engineered tissues that would be systems for drug screening and physiological testing."
Marius Wernig, who conducts research at Stanford, takes a different approach to reprogramming: transdifferentiation.
Instead of generating pluripotent stem cells in culture from adult somatic cells, and differentiating these into the different cell types of interest, Wernig changes one cell type directly to another. Indeed, because of his work he won the ISSCR's Outstanding Young Investigator Award.
I asked him if his approach started out as a shortcut to get around the more time consuming process of conventional stem cell development.
"Yes, it is a shortcut. But it's really a different process." With conventional induced pluripotent stem cells (iPSCs), he said, even if only a small fraction reprogram successfully, the cells grow very well.
"So, once you have a good iPS cell grown, you can expand it and fill up the incubator or the tissue culture in the incubator with these cells."
But you have to differentiate them again into the cell type you want. And that means another layer of time consuming work.
"It is the advantage of our approach to reprogramming --you don't have to go through this very laborious process in the first phase."
But it does have a drawback. "Once we have these cells (for example, neurons), we can't keep growing them anymore, as with the conventional approach. Especially neurons. So there is a limitation in expanding these types of cells."
But a main advantage will be in the clinic. Because transdifferentiation is a shorter process, it's easier to screen many more patient's cells --than with the conventional iPSC approach.
The goal: one day very patient-specific disease modeling and screening. Wernig said his lab now derives fairly homogeneous and fairly mature cell types in the transdifferentiation process.
He most recently tried transdifferentiating embryonic stem cells into neurons with a colleague. "And that worked beautifully."
"So, in two weeks, very pure, very homogeneous cultures of neurons--and the vast majority have synaptic ability, they make connections, talk to each other, and that is the feature of a mature neuron that was more difficult to achieve with the conventional iPSC approach."
The other approach to reprogramming is the one that most recently grabbed headlines. The cloning approach of Shoukhrat Mitalipov: generating an embryo by swapping a somatic cell nucleus into a human egg cell, or oocyte, and deriving a new line of pluripotent stem cells
In a sense, this is the most 'natural' approach to reprogramming --letting the egg cytoplasm do the work. And it has raised a number of scientific challenges, he said.
"At this point we're trying to understand the reprogramming 'machinery' in the process. It's very complex. There is a lot to learn about the oocyte and how it directs reprogramming."
But, as a recent post at Genetic Engineering and Biology News asked:
So now that other methods of making pluripotent cells have been demonstrated, why bother with SCNT? For one thing, human induced pluripotent stem cells (iPSCs) have a high frequency of copy number alterations, and reprogramming is often not complete. Dr. Mitalipov has begun comparing his NT-ESCs to iPSCs, as well as distributing them to other labs for them to study to see if they suffer the same fate. He has another, more direct, reason for creating NT-ESC cells: Dr. Mitalipov’s lab studies mitochondrial DNA, which as part of the cytoplasmic compartment remains in the oocyte while the nuclear compartment is completely replaced. Thus, SCNT replaces mutated mitochondrial DNA in a patient cell in NT-ESCs—which iPSCs do not accomplish.
What about the future of embryonic stem cell research? I posed this question to everyone on the panel. Is it still necessary given the advances made with iPSCs?
"Our understanding that iPS reprogramming can be done came from the study of embryonic stem cells," said Mitalipov. "And most of the knowledge about pluripotency came from embryonic stem cells. Embryonic stem cells have given us so much understanding of this stage of development."
Pluripotent cells are still experimental, he said. "Still a research tool. We're just trying to understand if they can be useful for therapies, and I think having more options is always better. For example, when screening for drugs, you start with a hundred candidates--only one may work. So, I think with stem cell therapy it's also good to have several candidate cell types and hopefully we can find the best.
"It's likely," he added, "that for different disease types, one cell type would be better than the other, and I think we have to continue to study embryonic stem cells as well as iPS cells."
Shinya Yamanaka, who won the Nobel Prize for first generating iPSCs agreed with Mitalipov. "It's definitely necessary for us to retry many research results using human embryonic stem cells," he told me. "Without them, later, we would never have developed induced pluripotent stem cells. So the answer is yes. It makes our research more solid."
The retro-viral approach pioneered by Yamanaka. The chemical approach which I written about before. Wernig's approach to direct transdifferentiation. And now the nuclear transfer --or cloning--approach.
The research toolbox has certainly grown for stem cell scientists in the last decade. And it looks like all these methods are going to be key to the future progress of getting drugs and therapies from the lab to the patient.
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