The Mechanical Cell

Researcher Don Ingber looks to art and architecture for inspiration on explaining disease

Photography: Mark Ostow                   

By Nancy Fliesler

One day in the 1970s, an undergraduate biology student at Yale named Don Ingber signed up for a three-dimensional design class. The course was normally restricted to art majors, but Ingber loved the designs he observed in nature and talked his way in.

The class was studying a work by contemporary sculptor Kenneth Snelson that was composed of long tubes that seemed to be suspended in midair but were actually attached by a network of tensed cables that stabilized the structure. When the professor pressed down on a model of the sculpture, it flattened, but when he released his hand, the sculpture sprang back to shape and bounced up into the air. Ingber's mind immediately flashed back to a science lab where he'd seen a similar thing happen with cells.

Snelson's sculptures rely on tension, rather than the force of gravity, to hold them together and create a stable form. Architect Buckminster Fuller dubbed this property "tensional integrity," or "tensegrity," and used it in his famous geodesic domes. Sitting in that art class, Ingber had an intuition that cells, too, were tensegrity structures. He has spent much of his adult life substantiating this idea, which many of his peers once viewed as preposterous.

Pushes and pulls
In 1984, Ingber graduated from Yale with an MD and PhD and joined the Children's Hospital Boston lab of Judah Folkman, MD. With Folkman, he has studied how mechanical cues drive and direct the growth of blood vessels, a process known as angiogenesis. But Ingber also has become a pioneer in mechanobiology - the study of how physical forces affect the function and behavior of living cells and tissues and, ultimately, explain disease. One of the core principles of mechanobiology is tensegrity.

Ingber argues that mechanical forces - pushes, pulls, tensions, compressions - are important regulators of cell development and behavior. Tensegrity provides the structure - the architecture if you will - that determines how these physical forces are distributed inside a cell or tissue, and how and where they exert their influence.

At the turn of the last century, scientists commonly described biological phenomena in terms of mechanics. "The early developmental biologists would watch embryos developing, which they saw as a mechanical process," Ingber says. "Cells were changing shape, pulling on each other, and moving." And when engineers examined human leg bone, they saw that its spongelike structure was laid out in patterns that allowed it to sense mechanical forces and arrange its tissue structure to optimally distribute the load. "That's the essence of design," says Ingber.

But this appreciation of mechanics and form fell away as the 20th century progressed. In the 1970s and 1980s, the field of molecular biology took hold. Scientists were focused on finding and mapping individual chemicals and genes as a way of understanding physiology and disease.

"Medicine went from a holistic view of describing the relation between form and function to a much more reductionist view of describing what life is made of," Ingber says. "And the mechanics were thrown out, like the baby with the bathwater."

The mechanical anatomy of a cell
In trying to reestablish a physical view of biology, Ingber has shown that cells, far from being formless blobs, use tension to stabilize their structure. And he has demonstrated, through two decades of experiments, that tensegrity not only gives cells their shape, but helps regulate their biochemistry.

Every cell, Ingber notes, has an internal scaffolding, or cytoskeleton, a lattice formed from molecular "struts and wires" not unlike the rigid tubes and tensed cables of Snelson's sculptures. The "wires" are a crisscrossing network of fine cables, known as microfilaments, that stretch from the cell membrane to the nucleus, exerting an inward pull. Opposing the pull are microtubules, the thicker compression-bearing "struts" of the cytoskeleton, and specialized receptor molecules on the cell's outer membrane that anchor the cell to the extracellular matrix, the fibrous substance that holds groups of cells together. This balance of forces is the hallmark of tensegrity.

Tissues are built from groups of cells, which Ingber likens to eggs sitting on the "egg carton" of the extracellular matrix. The receptor molecules anchoring cells to the matrix, known as integrins, connect the cells to the wider world. Ingber's group in Children's Vascular Biology Program has shown that a mechanical force on a tissue is felt first by integrins at these anchoring points, and then is carried by the cytoskeleton to regions deep inside each cell. Inside the cell, the force might vibrate or change the shape of a protein molecule, triggering a biochemical reaction, or tug on a chromosome in the nucleus, activating a gene.

Ingber says that cells also have "tone," just like muscles, because of the constant pull of the cytoskeletal filaments. Much like a stretched violin string produces different sounds when force is applied at different points along its length, the cell processes chemical signals differently depending on how much it is distorted.

"A growth factor will have different effects depending on how much the cell is stretched," says Ingber. Cells that are stretched and flattened, like those in the surfaces of wounds, tend to grow and multiply, whereas rounded cells, cramped by overly crowded conditions, switch on a "suicide" program and die. In contrast, cells that are neither stretched nor retracted carry on with their intended functions.

Location, location, location
Another tenet of cellular tensegrity is that physical location matters. When regulatory molecules float around loose inside the cell, their activities are little affected by mechanical forces that act on the cell as a whole. But when they're attached to the cytoskeleton, they become part of the larger network, and are in a position to influence cellular "decision-making." Many regulatory and signaling molecules are anchored on the cytoskeleton at the cell's surface membrane, in spots known as adhesion sites, where integrins cluster. These prime locations are key signal-processing centers, like nodes on a computer network, where neighboring molecules can receive mechanical information from the outside world and exchange signals. "Adhesion sites are what's important for major control of the cell," Ingber says. "If you're in one of these sites, you're hooked up to a bunch of players, both mechanical and chemical. You can affect these players, which in turn affect a bunch of other players."

Ingber offers the example of the oncogene src, one of the first genes known to cause tumors. This mutated gene doesn't shut off - it sends unrelenting chemical signals telling the cell to grow. "But what's interesting is that src is normally found on the cytoskeleton in the adhesion sites, near its signaling partners," he says. "To produce a cancerous transformation, it must be at these sites because it needs to be integrated within the structure of the cell."

Disease mechanics
Based on these observations, Ingber believes that genes and molecules only partially explain disease origins. In fact, he asserts that many medical conditions are caused by a mechanical failure at the cell and tissue level. Examples include congestive heart failure, where the heart muscle loses its elasticity and becomes "floppy," thus losing its pumping efficiency; and asthma, where changes in tissue mechanics cause the airway to stiffen, tighten and contract, increasing mechanical resistance and constricting breathing.

But often the mechanical basis of a disease is not so obvious. On an airplane not long ago, Ingber found himself sitting next to Jing Zhou, a researcher from Brigham and Women's Hospital, who told him about her work on polycystic kidney disease, or PKD. In children with PKD, huge cysts form in the kidney tubules, eventually replacing much of the mass of the organ itself, and causing the kidneys to fail. Zhou's lab had found a gene linked to PKD and localized it to a thin antenna-like structure sticking out of the kidney cell, known as the primary cilium. But she had no explanation for the finding.

Ingber pointed out that the cilium is designed to sense mechanical forces ¨ in the case of the kidney, the shear stress caused by urine flow. Normally, the force of the flow bends the cilium, triggering calcium to rush into the cell. He suggested to Zhou that perhaps cells affected by PKD have a faulty calcium signal and constantly "think" that shear stresses are high. This in turn might cause the tubules to enlarge more and more to accommodate the flow, eventually forming cysts. From this serendipitous meeting, a collaboration was born, and together, Ingber and Zhou showed that when the PKD-causing genes are disabled in mice, the "lever" of the primary cilium malfunctions and fails to trigger a normal calcium response.

Scientific heresy?
Ingber has worked hard to defend the notions of cellular tensegrity and mechanical forces regulating cellular biochemistry. He recalls being publicly attacked while presenting at scientific meetings. But he also remembers an eminent scientist telling him, "If you've got them that upset, you must be on to something important." And so Ingber returned to the lab bench. "I responded to my critics by devising experiments," he says.

In 1993, his team reported in Science that when they used magnetic forces to literally twist the integrin receptors at the cell surface, the cytoskeleton stiffened in response to the stress and behaved like a tensegrity structure. In 1997, the team reported in the Proceedings of the National Academy of Sciences that tugging on the same integrin receptors causes changes in the cell nucleus. In 2000, a study in Nature Cell Biology demonstrated that mechanical stress at the cell surface causes the release of chemical signals inside the cell that kick genes into action. Tweaking receptors not linked to the cytoskeleton had no such effect. Other experiments have altered the extracellular matrix - making it alternately rigid or flexible - and documented effects on cell signaling and gene expression.

Nanotechnology and beyond
Ingber's study of tensegrity's role in disease has helped him forge some unexpected connections. In 2003, he worked with Harvard physics professor Eric Mazur on a nanote chnology project, using a laser to obliterate a miniscule portion of a cell, a few billionths of a meter in size, without affecting surrounding structures. Ingber got involved because he sees the laser as a tool for cutting out a single structure in a living cell to explore its mechanical role. He has also delved into systems biology, a new field that uses computational approaches to explore how molecular parts organize themselves into a system whose properties cannot be predicted by the parts alone. Informed by tensegrity, Ingber hopes to understand how structural, mechanical, chemical and genetic factors combine to govern cell behavior.

He has also helped devise new approaches to tissue engineering, and even posits that tensegrity helps explain the origins of life. Observing that viruses, enzymes, cells, and even small organisms take geodesic forms like hexagons and helices, Ingber suggests that tensegrity is nature's way of creating strong, stable life forms with minimal expenditure of energy and materials.

"Tensegrity has given me a path that goes deep and broad," Ingber says. "I believe the greatest value comes when you cross barriers and boundaries, and get a new perspective and vantage point. I'm not afraid of following my own path."

To support vascular biology research at Children's
contact Joan Romanition in the Children’s Hospital Trust
at (617) 355-2420 or joan.romanition@chtrust.org