The field of stem cell research has continued to flourish with last month's approval of the first human embryonic stem cell-derived therapy to treat paralysis. As important a milestone as this is in the field, more work is needed to define the mechanisms by which embryonic stem cells become mature tissues, such as neurons.
This process of differentiation and maturation is dependent on the expression of specific proteins at specific times in the cell's development. Scientists have been interested in finding out more about these proteins, in the hope of one day being able to directly control their expression patterns, thereby forcing a stem cell to mature into a particular cell type.
A team from the medical school has pioneered a novel method to study the expression patterns of proteins in embryonic stem cells as they differentiate into motor neurons and astrocytes, a supporting cell in the brain. The results are published in the Journal of Proteome Research.
"We chose to study the development of embryonic stem cells into neural cells as these cells are potentially the first type of cells, except for bone marrow, that will be successfully implemented in stem cell-based therapies," Candace Kerr, an assistant professor at the medical school and a leading co-author of the study, said.
Stem cells are complex cells with two major defining properties: They can self-renew (make more of themselves), and they can become any cell in their lineage. There are many types of stem cells, but they can be put into the two categories of embryonic and adult stem cells.
Embryonic stem cells are derived from the cells of the inner cell mass of an embryo. Adult stem cells are tissue-resident cells that maintain adult tissues for the life of the organism.
The researchers looked at 1,200 different proteins at different time points as the embryonic stem cells were differentiated into motor neurons and astrocytes.
Previous research has only been able to look at four time points simultaneously, whereas this research could look at eight different time points during the differentiation process.
It is especially useful because it offers a novel temporal analysis of stem cell differentiation.
The method uses cells at each of the eight different time points, isolates the proteins inside each of the cells and then specifically labels the proteins with chemical modifiers. The proteins are then analyzed using a mass spectrometer, which separates molecules based on their size.
Once the protein sizes have been determined, the relative expression of each protein can be analyzed and compared to the other time points.
"The main question we asked was whether there were unique proteins that these cells express that can be used to?distinguish them?from different?kinds of?neural cells," Kerr said.
This research has identified new proteins involved in the differentiation process to motor neurons and astrocytes, as well as confirming proteins long thought to play a role.
Some proteins follow a trend of decreasing expression as the embryonic stem cells go to motor neurons. This is in contrast to other proteins that increase expression relative to embryonic stem cells.
The results retrieved from the new method were confirmed by using traditional methods such as western blotting and immunohistochemistry - both of which involve antibodies binding specifically to proteins.
Although this research method identified novel proteins involved in maintaining the embryonic stem cell as well as proteins involved in differentiation, it missed some proteins that have been shown to be critical for maintaining embryonic stem cells.
A drawback to this approach is that some transcription factors have a huge effect on the state of the cell but are expressed at low levels in the cell. Therefore, some important proteins could have been missed by this study because its sensitivity is still quite low.
However, the research does have major implications for the future of taking stem cells from the laboratory bench into clinical studies. "From this study, potential candidate markers can be targeted as potential ways in which to provide pure populations of neural cells for transplant therapy," Kerr said.
The recently approved clinical trial of embryonic stem cells could potentially require the information derived from this study. The new clinical trial uses oligodendrocytes, another type of nervous system tissue, to help repair the spinal cord. A first step before taking these cells to the clinic is making sure the cells are pure, or all the same kind.
"Isolation of pure populations of cells for any therapy use is not only important for?safety?concerns but also for optimizing their potential?success in treating such afflictions as ALS, Parkinson's, Alzheimer's?and Huntington's disease, to name a few,"?Kerr said.
It is the same standard required for drug-based therapies. For instance, if a drug is actually made up of two types of drugs, there could be adverse side-reactions. The same goes for cellular therapies. If the cell population is not pure, then that small subset of cells could go rogue and form a cancer inside the patient.
Future work in this field will look at more efficient ways to derive differentiated cells for stem cell therapies. This is a necessary hurdle that must be cleared before taking stem cell therapies into the clinic. Kerr said that this is "an issue which will be critical as millions of?cells and costly?culturing materials?are required to treat?a single patient."
