MIT chemists discover how a shape-changing receptor affects cell growth
This receptor, called epidermal growth factor receptor (EGFR), is overexpressed in many types of cancer. In fact, it is the target of several cancer drugs. Although these drugs often work well at first, tumors develop resistance to them. A better understanding of the mechanisms of these receptors may help researchers design drugs that circumvent this resistance, says Gabriela Schlau-Cohen, an associate professor in MIT’s Department of Chemistry.
“Thinking about the more general mechanisms that target the EGFR is an exciting new direction and gives you a new avenue to think about possible therapies that might not be as prone to evolving resistance,” she said.
Bin Zhang, assistant professor of chemistry career development at Schlau-Cohen and Pfizer-Laubach, is the senior author of the study, which was recently published inNature Communications” magazine. The paper’s lead authors are MIT graduate student Shwetha Srinivasan and former MIT postdoc Raju Regmi.
EGFR is one of many receptors that help regulate cell growth. It is present on most types of mammalian epithelial cells that line body surfaces and organs, and respond to several types of growth factors in addition to EGF. Some types of cancer, especially lung cancer and glioblastoma, overexpress EGFR, which can lead to uncontrolled growth.
Like most cellular receptors, EGFR spans the cell membrane. The extracellular region of the receptor interacts with its target molecule (also known as the ligand); the transmembrane portion is embedded within the membrane; and the intracellular portion interacts with the cellular machinery that controls growth pathways.
The extracellular part of the receptor has been analyzed in detail, but the transmembrane and intracellular parts have been difficult to study because they are more disordered and cannot be crystallized.
About five years ago, Schlau-Cohen began trying to learn more about these little-known structures. Her team embedded these proteins in a special self-assembled membrane, called a nanodisc, that mimics a cell membrane. She then used single-molecule fluorescence resonance energy transfer (FRET) to study how the conformation of the receptor changes when it binds to EGF.
FRET is often used to measure the tiny distance between two fluorescent molecules. The researchers tagged the nanodisc membrane and the end of the protein’s intracellular tail with two different fluorophores, which allowed them to measure the distance between the protein tail and the cell membrane under various conditions.
To their surprise, the scientists found that the binding of EGF resulted in a major change in the receptor’s conformation. Most models of receptor signaling involve the interaction of multiple transmembrane helices to bring about large-scale conformational changes, but the EGF receptor has only a single helical segment within the membrane and appears to be inactive without interacting with other receptor molecules This change occurred below.
“The idea that a single alpha helix could deliver such a large conformational rearrangement really surprised us,” Schlau-Cohen said.
To learn more about how this shape change would affect the receptor’s function, Schlau-Cohen’s lab collaborated with Bin Zhang, whose lab runs computer simulations of molecular interactions. This modeling, called molecular dynamics, simulates how a molecular system changes over time.
The model shows that when the receptor is bound to EGF, the extracellular part of the receptor stands upright, while when the receptor is not bound, it lies flat on the cell membrane. Similar to the closing of a hinge, when the receptor lies flat, it tilts the transmembrane segment and pulls the intracellular segment closer to the membrane. This prevents the intracellular region of the protein from interacting with the machinery needed to initiate cell growth. Binding of EGF makes these regions more available and helps activate growth signaling pathways.
The team also used their model to find that positively charged amino acids in the intracellular segment close to the cell membrane are key to these interactions. When the scientists mutated these amino acids to switch them from a charged state to a neutral state, ligand binding no longer activated the receptor.
Zhang Bin said: “We could see good agreement between simulations and experiments. Through molecular dynamics simulations, we could figure out which amino acids were critical for coupling and quantify the effects of different amino acids. Then Gabriela show that these predictions are correct.”
In addition, the researchers found that cetuximab, a drug that binds to EGFR, prevents this conformational change from occurring. Cetuximab has shown some success in treating patients with colorectal or head and neck cancer, but tumors can become resistant to it. The authors say that learning more about how EGFR responds to different ligands could help researchers design drugs that are less likely to lead to resistance.