Tiny physical pokes and prods may be enough to spur stem cell movement and differentiation, according to a new study published online in Nature Materials today (October 18). These findings suggest mechanical forces in the early embryo's microenvironment may play a bigger role in its development than scientists had realized.

A soft embryonic stem cell (left) has a large nucleus (blue) and relatively little actin (red) and responds to applied stress by spreading and downregulating gene expression; stiff differentiated cells (right) have more actin bundles and do not respond to applied stress. Image: F.Chowdhury/N.Wang

"I think it is very exciting work," Harvard University cell biologist Donald Ingber, who was not involved in the research, wrote in an email to The Scientist. "It further goes to show the fundamental role that mechanical forces play in development control."

Previous research has shown that stretching or stressing a whole cell can elongate the cell in the direction of the stress, but how cells would respond to relatively small and localized external forces was unclear. Bioengineer Ning Wang of the University of Illinois at Urbana-Champaign and his colleagues carefully attached a ligand-coated bead to a focal adhesion -- a complex on the cell surface that binds it to the extracellular matrix. They then used a magnetic field to gently push and pull on the bead, a technique known as magnetic twisting cytometry. Within 30 seconds of applying the force, mouse embryonic stem (mES) cells began spreading across the substrate. mES cell-differentiated (ESD) cells, however, did not seem to respond at all.

"We were so surprised," Wang said. "Every time we give a little bit of force, [the ES cells] start to spread, [but] then we look at differentiated cells, and they don't do that."

It turns out that mES cells are 10 times softer than ESD cells. The softness of a cell is determined primarily by the size and structure of the cell's cytoskeleton. Specifically, the polymerization of F-actin bundles during focal adhesion formation stiffens the cells. When the researchers artificially softened the ESD cells by growing them on a substrate only sparsely coated with the proteins needed to support the generation of actin bundles, the cells started to spread in response to the localized stress.

"It's really how soft the cell is [that] will dictate how sensitive cells will be to local force," Wang said. "[Because ES] cells are so soft, they are so sensitive to force." Thus, small local forces, such as those inflicted by neighboring cells in the growing embryo, may be active players in development.

In addition to spreading, the local stress appeared to initiate differentiation in mES cells, as measured by the downregulation of oct3/4 expression -- a common marker of undifferentiated cells. Twenty-four hours after 60 minutes of force application, oct3/4 expression had dropped by 35%, and 48 hours later was down to 50% of its original levels. Unmanipulated cells just micrometers away in the same dish, however, continued to express oct3/4, suggesting that physical deformation of cells may be a cell-specific way to induce differentiation.

"There's a ways to go," Wang admitted, "but certainly this is opening new directions." Understanding the mechanism underlying these changes, and determining what types of mechanical perturbations can cause differentiation into which cell types, are next on Wang's to do list.

"It takes large steps forward toward [understanding] development as well as stem cell application," Dennis Discher of the University of Pennsylvania, who did not contribute to the research, said in an email. "It will no doubt inspire a lot more work on various types of stem cells."


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