How a Substrate Affects a Stem Cell’s Fate

Much of the stem cell field is devoted to figuring out how to direct what stem cells turn into. (In stem cell terminology, the process of having a stem cell become a different type of cell is called differentiation.) By better controlling a stem cell’s fate, we can more efficiently turn the cell into a tissue type we need, such as skin cells for tissue grafts or different kidney cell types for bioengineering a transplantable kidney. While there’s a lot we don’t understand about the differentiation process, it’s becoming increasingly clear that one important component is the substrate that the stem cells are grown on top of.

Earlier this week a paper was published that helps emphasize this point. The study investigated how the stiffness of the substrate that the stem cells are grown on affects the differentiation, and memory, of the stem cells. The researchers basically found that when the stem cells (specifically human mesenchymal stem cells) were cultured on a rigid substrate for a while, they were more likely to turn into a bone-like cell type. On the other hand, if the cells were grown on a soft substrate, they were equally likely to become a bone-like or a fat-like cell type.

There have been similar studies previously, but what allowed these researchers to make this breakthrough is the substrate system they used. They utilized a substrate (specifically a hydrogel, which is a water-based gel) that could be changed from hard to soft by shining a UV light on it. Normally, when stem cells are transferred to a different substrate, the process is very disruptive (often fatal for many cells), and it can drastically (though temporarily) affect their ability to interact with the new substrate they’re placed on. The ability to eliminate the step of mechanically relocating the cells is what makes this UV-controllable substrate system ideal for this type of study.

When trying to figure out why stem cell differentiation is affected by the stiffness of their substrate, the researchers found that this may be related to the relocation of some key transcription factors. Transcription factors are proteins that are involved in controlling the gene expression in a cell. To access the cell’s DNA and function, transcription factors have to be in the cell’s nucleus. The researchers found that a transcription factor involved in bone differentiation (called RUNX2) may be key in substrate stiffness-related differentiation – when cells were switched from a hard substrate to a soft one, over time this transcription factor became deactivated, moving outside of the cell’s nucleus.

But this raises additional questions. For example, why did the activity of some transcription factors change when the stiffness was altered? And better yet, how can we best use this knowledge to better guide differentiation of stem cells to become target cell types?

It should be noted that in addition to the stiffness of a substrate, its composition is also important for affecting stem cell differentiation. This is actually what my graduate studies focused on – it’s becoming clearer that the types of proteins surrounding stem cells help guide them to become certain cell types. But this shouldn’t be too surprising because naturally cells are surrounding by specific types of proteins in specific areas of the body, helping define their identity.

So while it can be a challenge to turn stem cells into the types of cells we want, as we develop better tools to understand the differentiation process, and to more closely mimic the cells’ “natural” environment, stem cell applications may become increasingly feasible.

For further reading:

• Chun Yang, Mark W. Tibbitt, Lena Basta, and Kristi S. Anseth’s article “Mechanical memory and dosing influence stem cell fate” in Nature Materials
• Kerry Grens’s article “Stem Cells Remember Substrates“
• Teisha J. Rowland et al.’s article “Differentiation of human pluripotent stem cells to retinal pigmented epithelium in defined conditions using purified extracellular matrix proteins“
• Teisha J. Rowland’s blog All Things Stem Cell
• Teisha J. Rowland’s book Biology Bytes: Digestible Essays on Stem Cells and Modern Medicine