Doing the math for membranes

CAMBRIDGE, Mass. –Determining the mechanisms that shape biological membranes has long been a tricky business.

Like a factory assembly line, eukaryotic cells are organized into membrane-bound, functional compartments called organelles. For instance, the nucleus is the repository of genetic information and houses the machinery that creates the messenger RNA transcripts, which direct the synthesis of new protein. Secreted proteins are synthesized in a second organelle, the endoplasmic reticulum (ER), which are exported to the cell surface by a third organelle, the Golgi complex. All membrane bound organelles are characterized by dynamic changes in membrane structure that are closely coupled to the function of these compartments.

As reported last week in PNAS, Whitehead Fellow Paul Wiggins and a team of scientists from the California Institute of Technology and the University of California have taken a novel approach to determining the forces that shape biomembranes: Wiggins and coworkers have determined the forces shaping membranes from the shape of the membrane itself!

In principle, the approach is simple. “When you step on a scale, a small spring in the scale bends. The amount by which this spring bends tells you how heavy you are or what force is being applied to the scale,” Wiggins says. “Similarly with membranes, springs or forces cause them to bend. In a sense, we wanted to see if we could play the same game with the organelles of a cell–to take the observed structure and see if we can predict what forces are applied to give rise to the structure and essentially hold the structure together.”

The technique combines biological techniques with single-molecule physics and mechanical engineering to measure the forces on three-dimensional membrane shapes, and then try to match them with a general mathematical model. For the first time, the scientists prove that a mathematical model can be created to predict the external forces applied to the membrane to those detected by high resolution three dimensional images.

“You can visualize an incredibly rich set of cellular structures and work backwards to see what forces are applied to give rise to these structures,” says Wiggins. “In principle we’ve shown that calculations are possible -- that it’s possible to take three-dimensional structures that scientists have begun to capture in highly detailed images, and say something about the forces being generated.”

“We’re getting a more and more quantitative understanding of how the cell’s molecular machinery gives rise to their activities,” says Wiggins. “By creating a molecular scale we can determine the forces and physics behind the way things work.  We have the ability to watch the assembly line of interlocking proteins within the cell and visualize how they work individually and work together.”

Using an artificial membrane that closely resembles the membrane of a living cell the researchers investigated the dynamic forces of the cell’s membrane and organelles. They did so by employing so-called optical tweezers, “which use the force of a focused laser beam to trap and move parts of the cell,” says Wiggins. “In this case the optical tweezers bound a micron-sized glass bead to the cell’s membrane that was then trapped by a laser beam and positioned to measure the forces applied within the cell.”

This technology enabled the researchers to play tug-of-war with the membrane and exert forces in different ways, enabling them to analyze the behavior of single molecules and the response of cells when their structure is dramatically changed.

Calculating these forces provides insights to the detailed mechanisms underlying the internal scaffolding of the cell.

”For example, when mitochondria undergo oxidative stress, that usually leads to a change in the structure of the mitochondria–the specialized organelles often referred to as the powerhouses of cells,” says Wiggins. “There is a close link between the ability of the mitochondria to function and its structure. By relating structure to force, we can uncover the crucial factors that lead to the change in the structure of the mitochondria and other organelles as well.”

Studying these mechanisms also provides clues about the life cycle of membrane-bound viruses. When membrane bound viruses replicate, they leave the cell by “budding,” a process that encapsulates the viral particle in the cell’s membrane.

“We can better understand key parts of the life cycle of certain viruses including HIV by better understanding the host machinery involved in the process,” said Wiggins. “It’s a mechanical problem, but we don’t know the host mechanism that’s necessary for this budding process to work.”

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Paul Wiggins’ primary affiliation is with Whitehead Institute for Biomedical Research, where his laboratory is located and all his research is conducted.

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Lee, Heun Jin, et al. "Membrane shape as a reporter for applied forces." Proc Natl Acad Sci U S A, November 25, 2008.