Researchers at the UCLA Eli and Edythe Broad Center of Regenerative Medicine and Stem Cell Research have discovered that a metabolic molecule called alpha-ketoglutarate helps pluripotent stem cells mature early in the process of becoming adult organs and tissues. The findings, published online today in the journal Cell Metabolism, could be valuable for scientists working toward stem cell–based therapies for a wide range of diseases.
Pluripotent stem cells have the ability to create any specialized cell in the body, such as skin, bone, blood ornervous system cells—a process called differentiation. Because of that ability, scientists are studyingpluripotent stem cells to determine whether they can generate healthy tissues that could be used to treat people with conditions ranging from Alzheimer's disease to blindness.
But to coax pluripotent stem cells into any desired cell type, scientists have to find the right conditions and mixture of molecules to add to the stem cells to promote differentiation.
"One of the biggest challenges in our field has been to use pluripotent stem cells to efficiently create specialized cells that can carry out specific functions in the body," said Dr. Michael Teitell, the study's senior author and a member of the Broad Stem Cell Research Center. "Our findings may help overcome that challenge and let scientists more easily create cells to treat disease."
As they differentiate into specialized cells, pluripotent stem cells undergo a shift in their metabolism, and they begin converting sugars to energy more efficiently. Teitell and his colleagues wondered whether molecules involved in metabolism, or metabolites, might be more than just byproducts of this shift, and might actually help the stem cells differentiate.
To find out, they added a metabolite called alpha-ketoglutarate to a mixture of molecules that normally turns human pluripotent stem cells into nervous system cells. Within the first four days of the experiment, 5 percent to 40 percent more cells differentiated into neural cells than usual. The researchers saw similar results when they added alpha-ketoglutarate to other cocktails of molecules that are used to produce other cell types. The alpha-ketoglutarate, they found, sped up the process of differentiation.
"On its own, alpha-ketoglutarate probably wouldn't promote differentiation, but when you add it to other factors that propel the creation of specialized cells, it seems to accelerate this process," said Tara TeSlaa, first author of the new study and a graduate student in Teitell's lab.
Since alpha-ketoglutarate is known to change how genes are regulated by removing methyl chemical groups from the DNA in a cell, Teitell and TeSlaa suspected that the molecule was helping cells turn off genes related to pluripotency and turn on genes related to more efficient differentiation.
To test that theory, they added another chemical, succinate, to the stem cell mixtures. Succinate blocks the same DNA demethylation chemical reaction that alpha-ketoglutarate promotes. Indeed, the addition of succinate caused the stem cells to differentiate slower and less efficiently, which provided further evidence that alpha-ketoglutarate works by acting on genes.
"Until very recently, metabolites have been overlooked as a way to help pluripotent stem cells differentiate," said Teitell, professor of pathology and laboratory medicine at the UCLA David Geffen School of Medicine. "This work helps to change that view."
Teitell and TeSlaa think that others in the field will build upon their study by testing whether alpha-ketoglutarate improves a variety of stem cell differentiation processes. They are planning follow-up studies to find out exactly which genes alpha-ketoglutarate regulates and how it can promote differentiation in some situations.
Scientists confirm reprogrammed adult stem cells identical to embryonic stem cells
Researchers from the Vavilov Institute of General Genetics, Research Institute of Physical Chemical Medicine and Moscow Institute of Physics and Technology (MIPT) have concluded that reprogramming does not create differences between reprogrammed and embryonic stem cells. The results have been published in the journal Cell Cycle.
Stem cells are specialized, undifferentiated cells that can divide and have the remarkable potential to develop into many different cell types in the body during early life and growth. In addition, they serve as a sort of internal repair system in many tissues, dividing essentially without limit to replenish other cells. When a stem cell divides, each new cell has the potential either to remain a stem cell or become another a more specialized cell type, such as a muscle cell, a red blood cell, or a brain cell (Fig 1). Scientists distinguish several types of stem cells. Stem cells that can potentially produce any cell in the body are called pluripotent stem cells. There are no pluripotent stem cells in an adult body; they are found naturally in early embryos.
There are two ways to get pluripotent stem cells. The first is to extract them from the excess embryos produced during the in vitro fertilization procedure. But this practice is still controversial technically and ethically because it does destroy an embryo which could have been implanted. This is why researchers came up with the second way to get pluripotent stem cells – reprogramming adult cells.
The process of "turning on" genes that are active in a stem cell and "turning off" genes that are responsible for cell specialization is called reprogramming. This technology was pioneered by Shinya Yamanaka, who showed that the introduction of four specific proteins that are essential during early embryonic development could be used to convert adult cells into pluripotent cells. He was awarded the 2012 Nobel Prize along with Sir John Gurdon "for the discovery that mature cells can be reprogrammed to become pluripotent."(Fig.2).
Thanks to their unique regenerative abilities, stem cells offer potential for treating any disease. For example, there have been cases of transplanting retinal pigment epithelium and spine cells from stem cells. Another experiment showed that stem cells were able to regenerate teeth in mice. Reprogramming holds great potential for new medical applications, because reprogrammed pluripotent stem cells (or induced pluripotent stem cells) can be made from a patient's own cells instead of using pluripotent cells from embryos.
However, the extent of the similarity between induced pluripotent stem cells and human embryonic stem cells is still unclear. Recent studies highlighted significant differences between these two types of stem cells, although only a limited number of cell lines of different origins were analyzed.
Researchers compared induced pluripotent stem cells lines reprogrammed from adult cell types that have been previously differentiated from embryonic stem cells. All these cells were isogenic, which means they all had the same gene set.
Scientists analyzed the transcriptome – the set of all products encoded, synthesized and used in a cell. Moreover, they elicited methylated DNA areas, because methylation plays a critical role in cell specialization. Thorough study of changes in the gene activity regulation mechanism showed that reprogrammed and embryonic stem cells are similar. In addition, researchers came up with a list of the activity of 275 key genes that can present reprogramming results.
Researchers analyzed three types of adult cells – fibroblasts, retinal pigment epithelium and neural cells. All of them consist of the same gene set, but a chemical modification (e.g. methylation) combined with other changes determines which part of DNA will be used for product synthesis.
The type of adult cells that were reprogrammed and the process of reprogramming did not leave any marks, concluded scientists. Differences between cells that did occur were thought to be the impact of random factors. "We defined the best induced pluripotent stem cells line concept. The minimum number of iPSC clones that would be enough for at least one to be similar to embryonic pluripotent cells with 95 percent confidence is five," says Dmitry Ischenko, MIPT PhD and Institute of Physical Chemical Medicine researcher.
Clearly, no one is going to convert embryonic stem cells into neurons and reprogram them into induced stem cells – that would be too time-consuming and expensive. This experiment simulated the reprogramming of a patient's adult cells into induced pluripotent stem cells for further medical use. Even though this paper does not propose a method of organ growth in vitro for now, it is an important step in the right direction. Both induced pluripotent cells and embryonic stem cells can help us understand how specialized cells develop from pluripotent cells. In the future, they might also provide an unlimited supply of replacement cells and tissues for many patients with diseases that are currently untreatable.
Scientists discover a new kind of stem cell
Scientists at Michigan State University have discovered a new kind of stem cell, one that could lead to advances in regenerative medicine as well as offer new ways to study birth defects and other reproductive problems.
In the current issue of the journal Stem Cell Reports, Tony Parenti, lead author and MSU cell and molecular biology graduate student, unearthed the new cells - induced XEN cells, or iXEN - in a cellular trash pile, of sorts.
"Other scientists may have seen these cells before, but they were considered to be defective, or cancer-like," said Parenti, who works in the lab of Amy Ralston, MSU biochemist, cell and molecular biologist and co-author of the study. "Rather than ignore these cells that have been mislabeled as waste byproducts, we found gold in the garbage."
A great deal of stem cell research focuses on new ways to make and use pluripotent stem cells. Pluripotent stem cells can be created by reactivating embryonic genes to "reprogram" mature adult cells. Reprogramming mature cells into induced pluripotent stem cells, or iPS cells, allows them to become malleable building blocks that can morph into any cell in the body.
For example, if a patient has a defective liver, healthy cells could be taken from the patient, reprogrammed into iPS cells, which could then be used to help regenerate the person's failing organ. Taking cells from the same patient may greatly reduce the chance of the body rejecting the new treatment, Parenti said.
Prior to the discovery of reprogramming, scientists developed pluripotent stem cells from embryos. However, the embryo produces not only pluripotent stem cells, but also XEN cells, a stem cell type with unique properties. While pluripotent stem cells produce cells in the body, XEN cells produce extraembryonic tissues that play an essential but indirect role in fetal development.
Parenti and his team speculated that if the embryo produces both pluripotent and XEN cells, this might also occur during reprogramming.
The eureka moment came when Parenti discovered colonies of iXEN cells popping up like weeds in his iPS cell cultures. Using mice models, the team spent six months proving that these genetic weeds are not cancer-like, as previously suspected, but in fact, a new kind of stem cell with desirable properties.
Even more surprising, the team found that by inhibiting expression of XEN genes during reprogramming, they could decrease production of iXEN cells and increase production of iPS cells.
"Nature makes stem cells perfectly, but we are still trying to improve our stem cell production," Parenti said. "We took what we learned by studying the embryo and applied it to reprogramming, and this opened up a new way to optimize reprogramming."
The team wouldn't have made this breakthrough without the high level of collaboration and access to cutting-edge facilities at Michigan State, he added.
The next steps of this research will involve seeing if this process occurs in human cells. XEN cells have yet to be discovered in humans, but the possibility of their existence is a key focus of the field.
"It's a missing tool that we don't have yet," Ralston said. "It's true that XEN cells have characteristics that pluripotent stem cells do not have. Because of those traits, iXEN cells can shed light on reproductive diseases. If we can continue to unlock the secrets of iXEN cells, we may be able to improve induced pluripotent stem cell quality and lay the groundwork for future research on tissues that protect and nourish the human embryo."