Shape Becomes a Life or Death Matter
By William J. Cromie
The shape of the cells in your body determines whether they will live or die, Harvard researchers have discovered. Cells flattened like fried eggs have a much better chance of surviving than rounded ones shaped like cupcakes.
This fundamental finding contributes mightily to the understanding of normal growth and development. It also bears on a new way to fight cancers and on the feasibility of growing replacement organs from knee cartilage to livers.
"Cells of different shapes respond differently to the same signals from their surroundings," says Donald Ingber, associate professor of pathology at the Medical School. "They can, for example, roll up and form blood vessels, or they can die for no apparent reason. We are trying to determine how these decisions are made."
Human cells live surrounded by a dense matrix of fibrous proteins and sugars, created by the cells themselves. The matrix has been compared to packing used to protect fragile objects during shipment. And, until recently, biologists treated it that way, considering it mere filler that could be intellectually disregarded. Only in the last five years have they realized that the extracellular matrix is the source of signals that determine the fate of cells and the tissue and organs that they form.
When researchers detach cells growing in lab dishes from their matrix, most of them die. Blood cells are an exception because they must flow freely through arteries and veins.
Ingber and his colleagues at Children's Hospital did a series of clever experiments showing how shape provides the key to this life-and-death attachment. They broke up the matrix into tiny islands. Cells forced to stretch themselves flat to bridge such islands continued to survive and reproduce themselves. With the matrix so spread out that a cell could attach to only one small island, the cell retained a round shape and did not thrive. Many of them died.
"If chemicals in the matrix are the predominant reason for survival and growth, then cells would survive equally well whether attached to one large dot, or several smaller ones with the same total area," notes Christopher Chen '90, a graduate student in Ingber's lab. "What actually happened was that cells stretching across many small islands did much better. These results provide the first conclusive evidence that shape is key to survival."
Spot Welds in Cells
What actually happens when a cell spreads across a matrix? Molecules on the cell surface bind to various proteins in the matrix. This binding attracts different molecules inside the cell to the point of attachment.
"These sites are like spot welds where active molecules inside the cell cluster together," Chen explains.
For example, Ingber's team placed cells that form blood-vessel linings on a matrix containing a protein known as fibronectin. Cells stretched out and multiplied until they covered the matrix. Those that stretched to the proper degree then rolled themselves into tubes that became capillaries, the smallest blood vessels.
When the same type of cells touched only one island of matrix, they failed to change shape and they died.
Such death, known as apoptosis, is natural. "It's the way that cells regulate their population and preserve biological balance," Chen says. "You can imagine an overcrowding situation where cells get squeezed together until they become rounded." These cells die so that others can survive.
Special techniques developed by George Whitesides, Mallinckrodt Professor of Chemistry, and his students make such experiments possible. They allow matrix islands as small as one micron, or 1/25,000 inch, to be created. The eccentrically flattened cells that spread over them measure about 1/1,000- by-1/250-inch (25-100 microns). Chen, who received his Ph.D. in biomedical engineering this year from the Harvard-M.I.T. Division of Health Sciences and Technology, worked with Milan Mrksich, a postdoctoral fellow in chemistry, to construct the islands.
The Shape of Cancer
This new perspective on shape adds greatly to the understanding of how normal cells decide to die or to grow and differentiate into eyes, hearts, skin, or muscle. Such outcomes depend on what kinds of molecules sit in a matrix and become bound to the cells.
"Now that we've found the normal mechanism for survival, we can explore what goes wrong when things don't operate normally," Ingber notes. "In cancer, for instance, cells ignore the signal that tells them not to grow. Instead they grow into tumors."
Such knowledge might be used for novel cancer treatments. Researchers at Lawrence Berkeley National Laboratory in California have found that custom-made proteins introduced into the matrix can block certain types of cell binding. In one laboratory experiment, this strategy seemed to switch malignant breast cells into normal mammary cells.
Ingber and Chen are excited about the possibility of preventing cells from forming blood vessels that nourish tumors. Judah Folkman, Andrus Professor of Pediatric Surgery at Children's Hospital, proved that small tumors remain harmlessly dormant without a blood supply. Therefore, the ability to coax blood-vessel cells to commit suicide rather than roll up into capillaries might be a powerful way to arrest cancer.
"This kind of treatment may work on a variety of cancers, because all solid tumors depend on a flow of blood for growth," Chen points out. Drugs to arrest the growth of blood-vessel lifelines are now being tested in patients.
Manipulating matrixes to produce tailor-made tissues for transplantation or for repair of injuries looms as another possibility. Medical School scientists have already constructed animal bladders, windpipes, bone, and cartilage in the laboratory. They also are attempting to grow replacement livers, bones, and ligaments for humans.
"We are on the threshold of a new field -- tissue engineering -- in which a few cells can be grown into replacement tissue and organs of many kinds," observes Chen.
Pushing Around Genes
These biological transformations must involve genes, those twisted molecules that hold blueprints for building every living part of every animal and plant.
"The matrix sends signals into a cell through molecules on the cell's surface," explains Chen. "Such signals are somehow transmitted to genes. It is tempting to conclude that cell shape directly controls regulation of the genes, but there's not enough evidence for that yet."
But there is some evidence for it. Cells, many people are surprised to learn, have their own skeletons. A delicate network of solid fibrous proteins crisscrosses a cell's interior, supporting it like the framework of a building. Ingber has been a pioneer in showing that displacements of this lacy scaffolding transmit forces from one part of a cell to another.
Tugging on a surface molecule, for example, can instantaneously cause movements in all parts of a cell, including the core, or nucleus, where genes sit on stringlike chromosomes. Experiments done by an Ingber team earlier this year demonstrated that poking the outside envelope of a blood-vessel cell with a tiny protein-coated bead produces realignments of structures in the nucleus.
Such displacements might put proteins in contact with genes, thus switching them on or off. Studies at the Medical School and elsewhere suggest that this type of control may be necessary for normal development or, when lost, for tumor formation. For example, it could be important in determining whether breast cells develop normally or become malignant.
These experiments and discoveries paint a completely new picture of how the units that make up our bodies do their work. That picture radically changes basic knowledge about normal development and growth, and it raises the possibilities of unique treatments for diseases that result when growth and development go awry.
Copyright 1998 President and Fellows of Harvard College