NIH Grant Funds Lehigh Researchers' Exploration of Lipid Membranes

Two Lehigh researchers have been awarded a five-year, $1.5 million grant from the National Institutes of Health to examine lipid membranes and the method in which lipids and proteins travel in response to fluid flow. Aurelia Honerkamp-Smith, assistant professor of physics, and Damien Thévenin, associate professor of chemistry, also will test whether flow transport of a membrane protein triggers intracellular signaling in endothelial cells

In blood vessels, the manner in which cells flow through their environment regulates processes such as blood pressure, bone density, and neural growth. Yet, the molecular factors behind flow mechanotransduction, the processes through which cells sense and respond to mechanical stimuli by converting them to biochemical signals, remain unclear.

Endothelial cells are found on the inside of blood vessels, and the team is interested in a specific response these cells have to flow. Honerkamp-Smith and Thévenin are investigating a process that occurs when these cells feel a flow. They hope to identify the fundamental properties and principles that govern flow transport of membrane-linked proteins in model membranes and develop a model that predicts protein motion in physiological situations. When endothelial cells feel flow, they quickly start to produce nitrous oxide. The project will explore how the flow-mediated lateral transport of glypican-1 initiates this short-term flow response in endothelial cells. The researchers will also consider how lipid sorting by flow contributes to flow signaling in their model system and living cell membranes.

The interesting thing about glypican is that it is also located on the outside of the cell, and it is unknown how it activates the nitrous oxide synthase, which is on the inside, says Honerkamp-Smith. She theorizes that physiologically significant protein and lipid concentration gradients arise from physical interactions between fluid flow and complex membranes. This hypothesis is based on the premise that extracellular lipid-anchored proteins such as glypican-1 can be transported along the plasma membrane by external flow, with the aqueous portion of the protein acting as a molecular sail.

“With this particular protein, its structure is such that it is really good at acting like a little sailboat,” Honerkamp-Smith says. “It's got a large part that sticks out into the fluid surrounding the cell. Then it has a lipid anchor, and so this means if you apply a flow to the cell, this protein can very easily move a long distance along the surface of the cell. This lateral movement is something that hasn't really been studied before as a flow sensing strategy for cells. With this funding, we can try to figure out whether it is actually the lateral movement of the protein that is activating the flow response instead of some other type of interaction.”

To do this, her lab employs flow cells, a confocal microscope, and microfluidic setups to apply precisely the flow they want. They can then examine glypican movement in artificial membranes, as well as in cells. Studying a series of proteins produced by genetic manipulation in the Thévenin lab will let the team determine how altering those proteins changes their function.

“We build model proteins with increasing complexity with different membrane anchors and sizes that can be analyzed in vitro or directly in cells," Thévenin says. "Aurelia's lab then measures how they flow and analyzes the results based on fluid physics. This systematic approach allows us to first understand the basic principles behind protein transport on the cell membrane. Our main goal is to link this lateral movement to signaling events inside cells and changes in cell response.

“The overall research direction speaks to Aurelia’s hypothesis, which I think is conceptually fantastic. It is not easy to test experimentally but it makes the overall project even more stimulating and highlights the importance of collaborative work in addressing exciting and impactful questions.”

Ultimately, the researchers envision that the models they produce will apply to multiple cell lines and flow conditions, and will lay the groundwork for future research directions.

Story by Rob Nichols