Stem cells are a little like teenagers, full of potential but not sure what they’re going to be when they grow up. It’s that uncertain destiny that makes stem cells so exciting to scientists and physicians, who hope to someday use them for everything from spinal cord repair to organ regeneration. But corralling the uncertain power of stem cells requires learning how to push them toward a desired fate, convincing them to become bone cells or liver cells or neurons. Most laboratories have figured out ways to accomplish this goal with chemicals, exposing stem cells to growth factors and other signals that lead it down a particular developmental path. But there may be another way to play guidance counselor to an indecisive stem cell – changing its physical shape.
This process, called cell patterning, is a primary research focus of Milan Mrksich, professor of chemistry at the University of Chicago. On the surface, cell patterning sounds like a mix of science fiction and Play-Doh art: cells are grown on a plate stamped with a special mold that forces the cells to form a particular shape chosen by the researcher. Those shapes can be as simple as circles or squares of various sizes or as complex as flowers, stars, and pentagons. And far from being mere aesthetic lab trickery, this shape-shifting can have dramatic biological effects upon the cell, its underlying skeleton, and even the expression of particular genes.
In a paper published in PNAS earlier this month, Kristopher Kilian, a postdoctoral fellow in Mrksich’s laboratory, applied the cell patterning technique to a particular type of stem cells. Mesenchymal stem cells, harvested from bone marrow, are the slightly less ambitious cousins of the more-hyped pluripotent embryonic stem cells that can change into virtually any cell type. As multipotent cells, MSCs are generally restricted to one of three career paths: fat cells called adipocytes, bone cells called osteoblasts, or cartilage cells called chondrocytes. But despite being limited, those outcomes are potentially very useful therapeutically should scientists learn how to reliably control the differentiation of these cells.
So Kilian, with colleagues Branmir Bugarija and Bruce Lahn, tested out cell patterning on his supply of MSCs. The first experiments confirmed that size and aspect ratio mattered: when given larger or wider areas to grow, the stem cells preferred to become bone cells instead of fat cells. Kilian then kept the size of the stamp constant, but altered the shape, forcing the cells into either a “flower” with curved edges (top row above) or a “star” with sharp edges (bottom row). Both cell shapes were then grown in the same media – a “cocktail” of signals promoting fat cell or bone cell growth, and fates were chosen.
The results? “Flowers make fat and stars make bone,” Kilian summarized. “The view is that, when you introduce the cocktail, the cells are driven one of two ways…the geometry dictates which path the cell goes toward.”
Kilian then went under the hood of this shape-shifting effect and looked at how shape exerts its powerful influence. Looking at the cytoskeleton of cells of each shape, he detected an obvious difference (depicted in the image above) – where star-shaped cells had formed an orderly scaffolding of stress fibers around their perimeter, the flower-shaped cells formed less-organized internal structures. At the joints of that scaffolding, structures called focal adhesions form, and because the star cells have more clearly defined joints than flower cells, their focal adhesions are predictably larger.
“Focal adhesions are basically the footprints of the cell; how cells feel their surroundings and environment and tell the cell whether it wants to move or stay put,” Kilian said. “What’s becoming more clear, and hasn’t been appreciated in the past, is the importance of focal adhesions in differentiation of stem cells.”
Beyond structural changes, shaping the cells also activated a number of signaling factors known to push a cell toward developing into either fat or bone. A DNA microarray showed large differences in gene expression between the flower cells and star cells, a dramatic demonstration of how external environment can have strong effects on a cell, even down to the genetic level.
The capacity of stem cells to reprogram their gene expression according to shape also suggests that this effect isn’t just a curiosity, but a natural process. While growing up in its natural setting, a stem cell would be expected to experience different surroundings that influence its eventual fate – those destined to become fat cells might be in a more fluid, flexible environment, while budding osteoblasts are raised in a stricter, more rigid setting (there’s an education metaphor somewhere here…). Shape may thus predispose a stem cell to go one way or another, with signaling factors from neighboring cells contributing the final push.
Whether cell patterning can be applied to aid medical treatments using stem cells remains a question for the future, Kilian said. Eventually, scientists may be able to engineer structures that exploit this shaping influence to help drive stem cells toward a particular fate. But for now, it remains a fascinating trick of the laboratory.
“That’s why this is so fun – you can manipulate all these things, and people have it done it by adding different compounds and different drugs, and now we can do it with shape, which I think is pretty awesome,” Kilian said. “Keeping everything else equal, you can just modify the shape of the cell and change how it’s going to receive some of these signals.”
You can hear an interview with Kilian and Mrksich and view more photos from their laboratory at the University of Chicago News Office.
Kilian, K., Bugarija, B., Lahn, B., & Mrksich, M. (2010). Geometric cues for directing the differentiation of mesenchymal stem cells Proceedings of the National Academy of Sciences, 107 (11), 4872-4877 DOI: 10.1073/pnas.0903269107