The Power of Three
Each of our cells has a time to die. Programmed cell death, or apoptosis, helps keep our bodies healthy by ensuring that excess or potentially dangerous cells self-destruct.
One way cells know when to pull the plug is through signals received by so-called death receptors that stud cells’ surfaces.
Researchers studying a death receptor called Fas have now found that for immune cells to hear the death knell, a largely overlooked portion of the receptor must coil into an intricate three-part formation.
The findings revise scientists’ understanding of how these receptors work and provide new ways to consider tackling diseases that can develop when apoptotic signals go awry, including cancer and autoimmune diseases.
Transmembrane receptors like Fas have three parts: a region that protrudes outside the cell and receives signals, a region embedded in the cell membrane and a region that relays the signal for the cell to act upon. Until recently, scientists thought the middle portion was simply an anchor.
“We weren’t convinced the transmembrane region did anything,” said James J. Chou, professor of biological chemistry and molecular pharmacology and co-senior author of the study.
But hints had been emerging from Chou’s lab and from others that there was more to the story. For example, researchers discovered mutations in Fas’s transmembrane region that cause cancer.
“This suggested the region played a role in apoptotic signaling and made us want to look more closely,” said co-senior author Hao Wu, the Asa and Patricia Springer Professor of Structural Biology and professor of biological chemistry and molecular pharmacology at HMS and senior investigator at Boston Children’s.
Wu has for years been studying the family of receptors Fas belongs to, called tumor necrosis factor or TNF receptors. She wanted to know the atomic structure of Fas’s transmembrane region because it would tell her something about the region’s function.
So she approached Chou, a specialist in nuclear magnetic resonance (NMR) spectroscopy.
Chou’s lab had to devise a technique to better mimic a real cell membrane’s oily bilayer and coax the transmembrane proteins to assemble in a test tube. But in the end, the team got its atomic structure.
Wu and Chou had been scratching their heads over what they’d found when they analyzed the DNA of the transmembrane region: The sequences suggested that the transmembrane proteins had helix shapes and that they also contained the amino acid proline, which “is supposed to be a helix breaker,” said Wu. How could they both be true?
NMR allowed them to resolve the conundrum. The 3-D structure confirmed that the transmembrane region consisted of three identical protein helices wound together—and revealed that proline introduced a “tiny kink” that allowed the helices to pack more tightly.
“It’s critical for stable assembly of the trimer,” said Chou. To the authors’ knowledge, no one had seen proline do that before in transmembrane receptors.
Collaborating with TNF specialist Richard Siegel, clinical director and senior investigator at the National Institute of Arthritis and Musculoskeletal and Skin Diseases, the team found that the snapshot they captured represents the Fas transmembrane region’s structure after it has been activated. They learned that when the extracellular region receives a “time to die” signal, it triggers the transmembrane region to coil into formation. Only then can the signal pass into the cell.
“We didn’t know how the three parts of the receptor cooperate to produce the signal,” said Wu. “This study provides the missing link to show how they all work together.”
“The textbooks need to be revised,” said Chou. “The transmembrane region is crucial in connecting extracellular signal binding and the chemical modifications that occur inside the cell.”
Further work showed that the cancer-causing mutations researchers had spotted in the Fas transmembrane region deformed its delicate structure and prevented signals from passing through. One of the mutations struck proline itself.
“The effects of some of the mutations are so subtle, it’s unbelievable,” said Chou. “We wouldn’t have seen them without obtaining the high-resolution structure.”
A larger story
This new understanding of how Fas receptors work at the molecular level could ultimately improve scientists’ ability to treat certain conditions related to faulty apoptosis signaling.
If a patient has a transmembrane mutation in Fas that prevents their immune cells from self-destructing and allows tumors to grow, “You could conceivably go through other death receptors” to deliver the message, suggested Chou.
Alternately, if cells are self-destructing too readily, researchers might consider “inhibiting the receptor directly in this region” to reduce the death signal, said Wu.
The study could have even wider impact.
“Fas is really just a model system,” said Chou. “What excites me is that the discovery is likely to be applicable to the other members of the TNF receptor family,” such as DR5, currently being pursued as a drug target for killing pancreatic and colon cancers, and TNFR1, which may improve autoimmune diseases when suppressed.
Already, Wu and Chou are exploring whether the transmembrane portions of other TNF receptors share Fas’s three-pronged, proline-kinked structure.
“Understanding the mechanistic basis of how these receptors signal means we can screen for drugs and antibodies in a more informed way and can do other creative things to kill cancer cells and not healthy cells,” said Chou.