by Laura Buenning
Cell membranes can tell us a lot about “how things work” in the natural world.
Painstaking research decodes the membrane’s complex, interacting molecules made up of proteins, peptides, and enzymes. Sometimes the pace seems unbearably slow, but what we learn often inspires ideas for practical biomolecular tools (new signal receptors, for example) or brings clarity about diseases, says JDRD team leader Andy Sarles.
Sarles, Ph.D. graduate student Graham Taylor, and ORNL’s LDRD team leader Shuo Qian have singled out cholesterol in the cell membranes of nerve cells in the brain as a possible source for answers about Alzheimer’s disease. Cholesterol is prevalent in all cell membranes, Sarles says, most especially in nerve cells (neurons) in the brain.
Exactly what triggers Alzheimer’s and the ultimate death of the brain’s neurons is the subject of intense study. Research has shown that clumped fibrils formed by amyloid beta (Aß) peptides may be accumulating in such a way as to alter nerve cell permeability—specifically the cell’s ability to move calcium ions in or out through the membrane. How the disease’s progression is influenced by cholesterol or other elements within the membrane remains in question.
“Cholesterol, itself, is known to make the cell membrane more elastic,” Sarles says. “[Cells] are fluid structures to begin with; adding cholesterol to the membrane increases their fluidity. So we think it might actually help other molecules wiggle their way in and out of the cell. But, we know little about cholesterol’s role in Alzheimer’s.”
Sarles’ team evaluates the effects Aß peptides have on the permeability of two converging cell membranes that have been synthetically filled with cholesterol molecules. They will use a molded polymer tool Sarles invented to create artificial membranes between two simple water droplets submerged in oil. The method allows them to control the fine details of membrane composition, size, hydration, and other properties—historically a difficult task to accomplish.
“Cell membranes are two molecules thick; that’s 5 nanometers or five-billionths of a meter,” he says.
His technique takes advantage of the fact that oil and water don’t mix. Water droplets in oil remain spherical to minimize their contact with oil. Phospholipids, the membrane’s main component, coat the water droplets; one side of the phospholipid molecule being attracted to water, the other to oil.
By pinching together and releasing the sides of a small flexible mold that holds their solution they bring the coated droplets into contact. “The droplets don’t merge; instead they zip together at the interface, making a droplet-interface bilayer,” Sarles says.
A current passed through electrodes inserted into each droplet tells the team when ions are being passed through the membrane. Aß peptides introduced into one of the two cells provide information about their behavior when cholesterol is, or is not, part of the membrane structure.
Sarles technique adds a unique dimension to Qian’s current approach, using new neutron scattering techniques to examine multilayered stacks, “almost like onionskins,” of phospholipid cell membranes.
Single channel recordings and GISANS of amyloid-beta peptides in fully hydrated, unilamellar lipid bilayers
Andy Sarles, UT Mechanical, Aerospace, and Biomedical Engineering Department
Developing Grazing Incident Small-Angle Neutron Scattering for studying the interplay between amyloid beta peptide and cholesterol in lipid bilayers
Shuo Qian, ORNL Center for Structural Molecular Biology