Mechanically guided cell migration: less of a stretch than ever.

How do cells find their way into a wound, how can we tell them how to do this better, and why is it so hard to convince them to find their way into the inside of an engineered tissue? Although migration of cells in three-dimensional tissues determines prognosis in a great many pathologies, little is known about how cells decide which way and how fast to go in three dimensions, or about how they choose from among the broad array of factors recently identified as important to these decisions. These factors include extracellular matrix cues such as mechanical, structural, steric, and chemical guidance cues and their gradients; cell shape and structure; and mechanical and chemical interactions with neighboring cells (1). Understanding this decision-making would certainly be transformative for guiding treatments and tissue engineering, but the field is a long way from unraveling this. Isolating mechanical factors is a particular challenge, because cells and the extracellular matrix can both change dramatically in response to mechanical stimulus, and also adapt to one another (2–5). The work presented by Rouillard and Holmes (6) marks an important step forward, providing a clear map of how cells choose direction based upon the way a wound site is restrained, and narrowing down tremendously the mechanical factors that are involved. With simple in vitro and numerical models of a wound, they have separated, for the first time (to our knowledge), the effects of mechanical strain from factors such as steric guidance cues, microstructural fiber orientations, and chemical gradients. With this, they have identified, for the first time (to our knowledge), how cells can be guided to take a more direct path into an elliptical wound by providing mechanical constraint along the short axis. What do these new results mean from the perspective of the biophysical mechanisms underlying cell locomotion? Although the mechanics of cells crawling with lamellipodia in two dimensions has been established for some time, cells in three dimensions use a much richer toolbox of protrusions including filopodia, blebs, lobopodia, and pseudopods, and often switch among mechanisms based upon currently unidentified cues (1). Rouillard and Holmes (6) leave much still unanswered, including the mechanisms of locomotion and how they might change with anisotropic boundary constraints. However, a remarkable aspect of what they have achieved is that invasion efficacy is controlled in their model without changing the speed of locomotion. This suggests that their new system might be one in which the mechanism of locomotion stays constant and only the directional choices are altered. If so, this is a promising model for study of the mechanics of locomotion as well. Another exciting angle of this work is its dissection of the cellular mechanisms that can enhance wound seeding. At the cellular level, we know that that strain and mechanical restraints are powerful cues for directing how the cytoskeleton responds and how cells choose polarity. Stress or strain fields predict cytoskeletal rearrangements in response to mechanical loading and constraints in both two- and three-dimensional culture (7–10). The underlying principles are debated fairly narrowly and form the foundation for highly predictive models based upon strain-dependent association/dissociation rate constants for stress fibers, or stress ranges needed for stress fiber stability (11). At the level of whole cells, very elegant principles of elastic interactions can predict changes to polarity (12). Mechanical factors affect local stiffening, and myofibroblasts are known to alter their own stiffness and that of their surroundings to match one another (5) and cells form their own contact guidance cues through active contraction (13). These recursive factors are further intertwined because, at least in two dimensions, cell movement is guided by substrate mechanics (14). What do the simple and elegant results of Rouillard and Holmes (6) mean in the context of all of these convoluted and unwieldy factors? Such factors are all potential contributors to the observation that anisotropic boundary conditions can cue cells to choose a straighter path into a wound, and much work remains to be done to sort out their potential effects. The modeling of these and other factors performed by Rouillard and Holmes (6) was appropriately simple for their aims, but extending the work to provide a foolproof toolkit for cell guidance requires more work. Like so many other phenomena in three dimensions, cells within these wounds likely respond through a diversity of pathways based upon their local conditions, but here the ensemble effect is quite clear and the data provide a healthy culture environment for future models. In the meantime, this advance has some immediate and important implications for tissue engineering and wound healing. Although extending an in vivo environment involves countless additional signaling and vasculature effects, the work of Rouillard and Holmes provides possible cellular-level insight into long-known benefits of compression and surgical Langer’s lines in improving wound-healing outcomes. And we certainly know over which wound axis we will tension the next adhesive strip our kids request. Until then, these data themselves mark an important step forward in our migration through the tangled matrix of cell locomotion in three dimensions.