Understanding threat and the 'fight or flight' impulse

TCD is part of a team that helped isolate an essential component of the ‘fight or flight’ reaction, something that has helped our species to survive

IF YOU’VE EVER been in a potentially life-threatening situation – maybe a truck suddenly roaring towards you as you cross the road – you’ll know all about the “fight-or-flight” response. It rapidly diverts blood to your muscles and helps you fend off or (more likely in the case of an oncoming truck) move away from danger.

Now an international team, involving Trinity College Dublin scientists and led by Brian Kobilka at Stanford University, has worked out the active structure of an important protein in this process: the human beta-2 adrenergic receptor. This connects to the adrenaline we release when under threat and triggers action within our cells.

It does this by sitting in the thin fatty cell wall or membrane with one end outside and the other inside the cell.

This means the outside end of the receptor can sense what is happening – in this instance the adrenaline message – and then transmit the information to the cell’s interior.

The receptor is a type of “G-protein-coupled receptor” (GPCR) and there are many others throughout the body, explains Martin Caffrey, study co-author and professor of membrane structural and functional biology from Trinity’s schools of medicine and of biochemistry and immunology.

Understanding more about this one receptor should also increase our understanding of the other GPCRs, he says. “There are over 750 GPCRs distributed throughout the body with representatives in almost every cell type.” They play roles in our sense of sight and smell, in heart and lung function, in how we respond to hormones and neurotransmitters and in immunity and inflammation. “About a third of drugs on the market today target GPCRs.”

Working with these substances isn’t always easy. They are fragile and their dual locations inside and outside the cell walls mean different regions of the protein like different environments.

The researchers managed to catch their quarry by literally de-greasing the cell membranes with detergent, then using adrenaline-lookalikes as bait to connect to and fish out the beta-2 adrenergic receptor they were looking for.

Then they coaxed the receptor to form crystals within a specialised, toothpaste-like viscous “mesophase” material.

“These are very small very fragile crystals, a 10th to a 100th of a millimetre in dimension,” says Caffrey, who worked with Joseph Lyons and Dr David Aragão in Trinity on the project.

“We can see these little crystals growing, and we have to go in then and harvest those crystals very gently.

“But we have to be careful, because in that viscous mesophase we grow the crystals in, if we disturb it too much it dissolves, it disappears.”

The precious crystals then got shipped to the Argonne National Laboratories near Chicago, where the researchers fired powerful X-rays at the crystals to work out the structure of the protein.

The project involved plenty of experimentation to get the conditions right, but the findings now help to explain how the adrenaline-type molecule binds to the receptor and the kinds of shape changes it causes, notes Caffrey.

“This is a brick in the wall, it’s a contribution. We are beginning to understand how these molecules work at a fundamental level, and the beauty of the particular target we are working on, the beta-2 adrenergic receptor, is that it is representative of a much bigger family,” he says.

“You cannot study all of them at once so choosing a good representative model, a paradigm, will shed light on the others and give some insight about how they work. And we are beginning to look at other GPCRs, we have targets we are working on right now.”

Trinity’s membrane structural and functional biology group is funded through Science Foundation Ireland, the US National Institutes of Health and the European Union Framework Seven Programme.