The Pearling Effect


You are here

From left to right: Prof. Samuel Safran, Dr. Alexander Bershadsky and Prof. Elisha Moses

A highlighted sentence in a book on the desk of Prof. Elisha Moses reads: "When the real world is recalcitrant, we build ourselves toy models in which the equations are simple enough for us to solve." Accordingly, Moses built a simplified model of a cell -- similar to a soap bubble but one-thousandth of a millimeter in size. Using this model, he has found out that cells can be tickled to "tears."
Moses, of the Physics of Complex Systems Department, agitates artificial cell membranes with laser tweezers (used for "grabbing" microscopic targets and moving them around). Along with Ph.D. student Roy Bar-Ziv, he found that pulling on membranes causes them to break into droplets reminiscent of a trail of tears or a string of pearls. Since cell shape is related to function, this finding is of great interest to biologists. So Dr. Alexander Bershadsky of the Molecular Cell Biology Department joined the research effort. Their aim: to find out if the "pearling" effect was present in live cell membranes as well.
And sure enough, Bershadsky was able to induce this effect in live cells through the use of a drug that virtually knocks out the structure inside the cell just below the membrane, called the cytoskeleton. Proof that the pearling effect also exists in live cells has lent additional weight to the research. Since weakening of the rigidity of the cytoskeleton is a characteristic of cancer cells, knowing the effect of this trait on membrane shape -- and how drugs control membrane shape -- may provide new clues to the development of cancer.
An almost inconceivable research trio came into being when theoretical physicist Prof. Samuel Safran of the Materials and Interfaces Department joined the team. For those not familiar with the scientific spectrum, biology and theoretical physics are considered to be at opposite ends. Very seldom does one find a biologist working alongside a theoretical physicist. "Biologists and physicists speak different languages. Our first objective was to find a common one. This study shows that we succeeded," says Safran.
Safran's research group constructed a theory that successfully predicts how the shape of the membrane will vary with cytoskeleton rigidity, and hence with the concentration of the drug. Says Safran: "The ability to predict is a major step toward the ability to control."
In addition, Moses is currently involved in another study combining physics and biology; he is investigating how neurons branch out and connect to one another. Neurons, which form a network through which they send electrical currents to one another, may be the key to constructing biological computer chips to replace or improve silicon chips in the future. Here the physicist must use his understanding of organization, connectivity and cooperation in many body systems to try to decipher the cell's innate ability to construct electrical circuits.
"This isn't the physics of the Big Bang," says Moses. "It's physics on a human scale, the physics of people."