The future of regenerative medicine continues to depend on the type of stem cell that is proven most effective, most scalable, and most time and cost efficient to produce. Approaches include embryonic stem cells, adult stem cells, Induced pluripotent stem cells, from both autologous and allogeneic sources. One of our Sector Companies, International Stem Cell Corp., is attempting to create a stem cell bank based on parthenogenic stem cells derived from unfertilized eggs (oocytes). Such a bank would have the following advantages:
- A scalable bank would contain a manageable number of stem cell lines that will be immunological matches for large patient populations of different ethnic origin.
- Unlike induced pluripotent stem (iPS) cells these would not involve extensive gene manipulation, which to this point in time have largely unknown biological impact.
We point this out in order to ask ourselves what the potential impact on such a stem cell bank might be from new study results obtained by researchers at Stanford University. Since human iPS cells were first created in late 2007, the emphasis has been on developing these cells in a faster, cheaper way that also provides safe and predictable biological results when used in regenerative therapies.
But what if the iPS phase can be skipped entirely? Will adult cells converted, for example, directly into nerve cells be an eventual solution to Parkinson's Disease? Or will parthenogenic, or some other more basic, more embryonic-like, cell provide the answer. Perhaps both will be a solution and success in the market, as well as in therapy, will be a function of efficiency from origin to therapeutic use, and resultant lower cost. Perhaps neither will be universally useful in general therapy, but will be more effective for therapies in particular parts of the body. At this stage no one knows for sure. What we do know is that knowledge is expanding rapidly as shown in this current Stanford study.
Stanford researchers have succeeded in transforming mouse skin cells in a laboratory dish directly into functional nerve cells with the application of just three genes. The cells make this transition without first becoming a pluripotent type of stem cell.
“We actively and directly induced one cell type to become a completely different cell type,” said Marius Wernig, MD, assistant professor of pathology and a member of Stanford’s Institute for Stem Cell Biology and Regenerative Medicine. “These are fully functional neurons. They can do all the principal things that neurons in the brain do.” That includes making connections with and signaling to other nerve cells — critical functions if the cells are eventually to be used as therapy for Parkinson’s disease or other disorders.
Although previous research has suggested that it’s possible to coax specialized cells to exhibit some properties of other cell types, this is the first time that skin cells have been converted into fully functional neurons in a laboratory dish. The change happened within a week and with an efficiency of up to nearly 20 percent. The researchers are now working to duplicate the feat with human cells.
“This study is a huge leap forward,” said Irving Weissman, MD, director of Stanford’s Institute for Stem Cell Biology and Regenerative Medicine. “The direct reprogramming of these adult skin cells into brain cells that can show complex, appropriate behaviors like generating electrical currents and forming synapses establishes a new method to study normal and disordered brain cell function. Finally we may be able to capture and study conditions like Parkinson’s or Alzheimer’s or heritable mental diseases in the laboratory dish for the first time.”
Until recently, it’s been thought that cellular specialization, or differentiation, was a one-way path: pluripotent embryonic stem cells give rise to all the cell types in the body, but as the daughter cells become more specialized, they also become more biologically isolated.
This view began to change when Dolly the sheep was cloned from an adult cell in 1997, showing that, under certain conditions, a specialized cell could shed these restrictions and act like an embryonic stem cell.
And it changed absolutely with the creation of induced pluripotent stem cells, or iPS cells, from human skin cells by infecting them with four stem-cell-associated proteins called transcription factors. Once the cells had achieved a pluripotent state, the researchers coaxed them to develop into a new cell type. The process was often described in concept as moving the skin cells backward along the differentiation pathway.
Finally, in 2008, Doug Melton, PhD, a co-director of Harvard’s Stem Cell Institute, showed it was possible in adult mice to reprogram one type of cell in the pancreas to become another pancreatic cell type by infecting them with a pool of viruses expressing just three transcription factors.
As a result, Dr. Marius Wernig began to wonder whether the pluripotent pit stop was truly necessary. To test the theory, Wernig, Thomas Vierbuchen and graduate student Austin Ostermeier amassed a panel of 19 genes involved in either epigenetic reprogramming or neural development and function. They used a virus called a lentivirus to infect skin cells from embryonic mice with the genes, and then monitored the cells’ response. After 32 days they saw that some of the former skin cells now looked like neural cells and expressed neural proteins.
The researchers, used a mix-and-match approach to winnow the original pool of 19 genes down to just three. They also tested the procedure on skin cells from the tails of adult mice. They found that about 20 percent of the former skin cells transformed into neural cells in less than a week. That may not, at first, sound like a quick change, but it is vast improvement over iPS cells, which can take weeks. What’s more, the iPS process is very inefficient: Usually only about 1 to 2 percent of the original cells become pluripotent.
In Wernig’s experiments, the cells not only looked like neurons, they also expressed neural proteins and even formed functional synapses with other neurons in laboratory dish.
“We were very surprised by both the timing and the efficiency,” said Wernig. “This is much more straightforward than going through iPS cells, and it’s likely to be a very viable alternative.” Quickly making neurons from a specific patient may allow researchers to study particular disease processes such as Parkinson’s in a laboratory dish, or one day to even manufacture cells for therapy.
The research suggests that the pluripotent stage, rather than being a required touchstone for identity-shifting cells, may simply be another possible cellular state. Wernig speculates that finding the right combination of cell-fate-specific genes may trigger a domino effect in the recipient cell, wiping away restrictive DNA modifications and imprinting a new developmental fate on the genomic landscape.
“It may be hard to prove,” said Wernig, “but I no longer think that the induction of iPS cells is a reversal of development. It’s probably more of a direct conversion like what we’re seeing here, from one cell type to another that just happens to be more embryonic-like. This tips our ideas about epigenetic regulation upside down.”
Adapted from the Stanford University School of Medicine Announcement
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