November 22nd, 2011

At a Single-Molecule Level, Laying to Rest Cell Signaling Disputes

By msequeira

Researchers at the UNM Cancer Center have put to rest several disputes about the activities of epidermal growth factor (EGF) receptors on the cell surface that help transmit signals involved in cell growth and survival. Gone haywire, these receptors can give rise to various cancers and other diseases. In a paper published in the November issue of Nature Structural and Molecular Biology, UNM Cancer Center scientists Diane Lidke, PhD, Shalini T. Low-Nam, PhD, and colleagues shed new light on EGF receptor interactions at a single-molecule level, thanks to powerful new imaging methods and mathematical modeling.

Cells respond to their environment by binding to chemical cues. One such cue common throughout the human body is EGF, a protein that helps regulate cell growth, proliferation and differentiation. EGF binds to complementary sites, known as EGF receptors, on the surface of target cells, initiating a complex chain of events that fosters cell growth and division. Mutations that affect the expression or activity of EGF receptors can result in the uncontrolled cell growth that is the hallmark of cancer.

In their recent study, UNM Cancer Center scientists observed in minute detail the activities of the EGF receptor, also known as erbB1, in the membranes of living cells. Specifically, they studied a process called dimerization, which occurs when receptors on the cell surface encounter each other, interact and form temporary bonds. The resulting receptor units are known as dimers. Dimer formation is related in complex ways to the binding of proteins (known as ligands in this context) to their complementary receptors. Together, ligand binding and dimerization play crucial roles in initiating cell signaling – helping receptors do their job within the cell.

While its importance is well established, certain key aspects of erbB1 dimerization are controversial or poorly understood. “The question we had,” said Dr. Lidke, “is what promotes dimerization, and how do various factors like the concentration of receptors, the availability of ligands and the organization of the cell membrane come into play?” Using sophisticated fluorescence imaging techniques, the team was able to capture a single-molecule view of erbB1 activity within cell membranes, providing unprecedented insights into how these receptors interact with ligands and with each other. They’ve made three important discoveries.

First, they’ve found that the diffusion, or mobility, of erbB1 within the cell membrane is more complicated than previously realized, reflecting a wide range of factors related to dimer status, ligand binding and changes in the local membrane environment. Understanding receptor mobility is crucial to understanding signaling because changes in mobility are linked to how chemical cues get transmitted from outside to inside the cell.

Second, they’ve discovered that dimers consisting of two EGF ligands and two EGF receptors are the most stable formation – and the most essential to the propagation of signals through the cell membrane. It’s important to realize that dimers form in all possible combinations of liganded and unliganded receptors, some combinations more stable and longer-lived than others, and that dimers are in a constant state of flux as receptors couple and uncouple repeatedly in response to various factors. The team’s findings related to the rise and demise of dimers will help update existing mathematical models of erbB1 signaling.

Finally, their work addresses the role of membrane organization in promoting signal initiation. Cell membranes consist of distinct domains, or compartments; the team’s study found that receptors confined with a given domain are more likely to encounter each other and form dimers, perhaps doing so repeatedly. The mobility and stability of dimers within domains has an effect on how efficiently signals are transmitted, and weak signaling may actually prompt the rearrangement of the membrane for better signaling advantage.

“The new quantitative methods described here capture dynamic receptor interactions at the single-molecule level, providing details that are obscured using traditional methods,” the investigators conclude in their paper. “Because dimerization is a common mechanism for signal initiation, our approach can be applied across many receptor systems.”

By clarifying the role of dimerization in normal erbB1 signaling, the UNM Cancer Center team is shedding light on the runaway signaling that can give rise to cancer. This in turn lays the groundwork for better understanding how certain cancer therapies work – and for developing new therapies that more precisely and effectively target abnormal signaling.

Paper reference

ErbB1 dimerization is promoted by domain co-confinement and stabilized by ligand binding” was published in the November 2011 issue of Nature Structural & Molecular Biology (volume 18, number 11) and appeared online in advance of print on October 23, 2011. Authors include Shalini T. Low-Nam (UNM Cancer Center, UNM Department of Pathology), Keith A. Lidke (UNM Cancer Center, UNM Department of Physics), Patrick J. Cutler (UNM Cancer Center, UNM Department of Pathology), Rob C. Roovers (Department of Biology, Utrecht University, The Netherlands), Paul M. P. van Bergen en Henegouwen (Department of Biology, Utrecht University, The Netherlands), Bridget S. Wilson (UNM Cancer Center, UNM Department of Pathology) and Diane S. Lidke (UNM Cancer Center, UNM Department of Pathology).

Tags: cell biology, Press Release Archive 2011, publication