One of the most astonishing characteristics of glass is that it can be as hard as, well, glass--yet malleable enough to assume the form of everything from a filigreed wisp to a womb-like hollow. Now new research led by Jeffrey Fredberg at the Harvard School of Public Health reveals--transparently--that our cells can essentially do the same.
Fredberg's research, published in the July issue of Nature Materials, shatters conventional notions about how cells go about their routine business of stretching, spreading, and contracting. The findings could have far-reaching implications for treating illnesses caused by mechanical dysfunction, such as inappropriate airway narrowing in asthma, cell invasion in cancer, and vessel constriction in vascular disease--and even for stopping infections.
"Jeffrey has overturned the established dogma regarding a model of the physical properties of mammalian cells," says Joseph Brain, Cecil K. and Philip Drinker Professor of Environmental Physiology at HSPH, and the former chair of Fredberg's department, that of Environmental Health. "Every scientist dreams of taking something everyone thought was true and replacing it with something better."
BURSTING A BUBBLE
For more than 50 years, scientists likened the cell to "a water balloon filled with honey, or sometimes, a gel"--an elastic membrane holding a viscous fluid, says Fredberg, professor of bioengineering and physiology at the School. But now Fredberg and his international team have used molecular-scale technology, or "nanotechnology," developed at HSPH to burst that bubble. Instead, they show that cells behave like molten glass, constantly deforming and rearranging their molecules while maintaining peak function. Just as cold glass becomes malleable when heated to a designated temperature, cells alter their form by shifting their "effective temperature"--a measure of molecular rearrangements.
CELLULAR SKEINS These pictures depict different components of a cell's cytoskeleton--the network of protein filaments that defines a cell's shape and confers it's mechanical properties. At HSPH, Jeffrey Fredberg's research showed that the way these structures deform (stiffen or relax), flow (become more or less viscous), and remodel is strikingly similar to the way glassy materials behave. Photos, Rick Rogers, Bioimaging Laboratory, HSPH
"The cell is a strange intermediate form of matter that is neither solid nor fluid, but retains features of both," says Fredberg, who trained as a mechanical engineer specializing in fluid dynamics, including how oxygen and carbon dioxide flow in the lungs. In response to need, he adds, or pathology, "the cell can vary its position along the spectrum between the two states."
The discovery sheds new light on the possible causes of asthma, which strikes when the smooth muscle surrounding the airways constricts inappropriately. In Fredberg's scenario, the cells embedded in that muscle become more solid-like, forcing the airways to narrow and then get stuck in that narrowed state (though why they get stuck isn't yet known). Conversely, a white blood cell that needs to crawl through an artery wall to chase bacteria in tissue makes itself more liquid-like to accomplish the task. This increased understanding of the mechanisms of diseases lays the groundwork for developing cures.
"The old school way of thinking is that cell structure is fixed," says Chun Y. Seow, who wrote the commentary that accompanied the Nature Materials paper, speaking by phone from the University of British Columbia, where he's an associate professor in the Department of Pathology and Laboratory Medicine. "That is, once a cell is fully differentiated--once it has become, for example, a smooth muscle cell as opposed to a skin cell--the scaffold that maintains the cell shape doesn't change anymore. But Fredberg is saying that cells do not have a fixed structure. That concept is new."
In their investigations, Fredberg's team--including James P. Butler, senior lecturer, and Ben Fabry, then a postdoctoral fellow with Fredberg and now professor of bioengineering at the University of Erlangen-Nurnberg, in Germany-- focused on smooth muscle cells, which are embedded in the walls of all hollow organs, including the bladder, the airways, and the veins and arteries. Whereas striated muscle, like that in a bicep, is responsible for motion, smooth muscle in these organs is charged with maintaining shape and tone.
The scientists put smooth muscle cells from human airways in a culture dish, and attached tiny magnetic beads to the cells' "cytoskeleton"--the network of protein filaments that defines the cell's shape and confers its mechanical properties--"like you'd sprinkle chocolate chips on top of cookies," says Fredberg. They then trained oscillating magnetic fields on the beads, at varying frequencies, to make them wiggle. The dancing beads, in turn, jiggled the cytoskeleton.
By measuring those vibrations, the researchers painted a panorama of the cell's mechanical properties. Surprisingly, instead of stiffening relatively suddenly as the frequency increased, just as the water-balloon model would predict, the cell's cytoskeleton gradually hardened, much as heated glass particles do in their frustrated search for molecular order.
Next, Fredberg and his colleagues attached the beads to the cytoskeleton and simply observed the results through a microscope--sans oscillating magnetic fields to juice them. Lo and behold, the tiny beads moved spontaneously. It was indisputable proof that the cytoskeleton was continuously remodeling itself. The revelation, however, was the nature of its movement: The beads didn't just amble; they hopped.
"At first it looked as though the beads were doing what's called Brownian motion--it's kind of a random drunkard's walk," says Fredberg, rattling his head and body in his chair to illustrate. "If you looked carefully at their traces, though, you saw a random walk, and then a hop, hop, hop. These hopping events are analogous to what people have found in glassy systems."
The hopping, it turned out, reflected the fundamental structure of the cell. Whereas the classical model imagined molecules swimming within the cytoskeleton in a kind of soup, the HSPH experiments showed the cells' molecular innards to be highly concentrated. "The molecules are jammed together cheek-by-jowl, like people stuck in an extremely crowded subway," says Fredberg. In this state of "kinetic arrest," he says, they're trapped, but not totally still: Just like the folks in the packed train, they're jostling their neighbors. And sometimes, if there's enough activity, an individual molecule will abruptly hop out of its trap only to fall into another--remodeling the system in the process and reflecting the transformation from solid to fluid. In the world of glassy materials, these phenomena have accepted names: rejuvenation (the hopping) and aging (the settling down).
Already pharmaceutical companies are using the HSPH team's work to speed the search for new drugs for asthma. Until now, new compounds have been tested as potential treatments in animals, and then in people--a laborious and expensive process. But with the HSPH nanotechnology and knowledge, researchers can screen compounds in a Petri dish to see how human smooth muscle cells react mechanically. Do the cells relax? Freeze up? Become "hot," fluid, and agitated? The advance enables drug companies to rapidly screen large numbers of molecules quickly and inexpensively. Regarding cancer, the research indicates that the cells in nonaggressive cancers are more solid-like, hence immobile (they're considered "cold"), while those in aggressive cancers are more liquid-like; they can move rapidly and invade tissue (they're "hot"). In the future, scientists may be able to detect how invasive various cancers are by taking biopsies and determining the cells' "effective temperature."
"Jeffrey's model is a Holy Grail, one many seek but few find," says Joseph Brain. "His work takes diverse complex phenomena and abundant data, and reduces them to one elegant equation, with which he can calculate the behavior of all kinds of cells under a variety of conditions. When everything falls into place like that, the model is generally right."
Thea Singer is a senior writer for the Review within HSPH's Office for Resource Development.