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