One on One: Cell talk
Cells bind to and communicate with the extracellular matrix via transmembrane integrins, enabling cells to respond to changes in their environment. To increase integrins’ affinity for ligand, the cell induces a process called “activation.” The details of how activation occurs, however, have been a mystery. Do integrins need to cluster within the membrane, or can they be directly activated by the cytoskeletal protein talin, without any clustering? Martin Humphries of the University of Manchester and F1000 Faculty Member in Cell Biology reviewed a recent paper that employed a clever tool to finally settle the debate. (J Cell Biol 2010 188:157–73, 2010).
TS: What is integrin activation?
MH: Integrins are the key receptors for the structural proteins outside of the cell. They’re there to transfer information from the immediate environment of the cell into the cytoplasm and ultimately the nucleus, to control how cells respond to their environment. So the cell needs a mechanism to sense that environment, but also to switch the receptors off and on.
TS: So cells can turn adhesion—the process by which they communicate with the outside matrix—on and off by activating integrins. Why is this so important?
MH: Without a mechanism to coordinate the interactions between different cells you can’t have a multicellular existence. Signaling is not all about growth factors and hormones—there’s a physical connection between cells that cells use to control their behavior, whether that’s movement or differentiation or division. The ligand-integrin-talin connection underpins multicellularity.
TS: There’s been a fierce debate about the role of clustering in integrin activation. What’s that about?
MH: Some labs have felt, over the last couple of decades, that the activation process really revolves around bringing receptors close together so you kinetically create conditions for increased ligand binding. Then there are other camps, of which I’ve been an advocate from the very beginning, that conformational changes, integrin shape changes, dominate the activation process. It’s now I think been proven incontrovertibly that conformation is critical to integrin activation.
TS: Why did you choose to evaluate this paper?
MH: What we need to do is narrow down what is a very complex process into a number of defined steps. We know a lot about the binding of ligands to integrins, we know a lot about the shape change within the integrin molecule, and we know a fair amount now at the structural level about the binding of the cytoplasmic domains of integrins to talin. But from that point on, the whole adhesion complex explodes into complexity. What this paper does is firm up one of the connections. It shows that binding of talin really can trigger the extracellular changes in affinity [of integrin for ligand]. The other reason the paper is important is that the methodology is now established to further elaborate the mechanism.
TS: Let’s talk about that methodology. To model events in the cell membrane, Mark Ginsberg at the University of California, San Diego, and colleagues created “nanodiscs”—tiny artificial membranes just large enough to contain a single integrin molecule. These were crucial to the experiment.
MH: Yeah, they’re the clever part. [The authors] combined it with electron microscopy to demonstrate that there is only a single integrin in the discs. That’s how they imaged the [conformational change, a sign that activation has occurred] in the integrin, by single molecule EM.
TS: Are there any potential pitfalls in this approach?
MH: There’s a danger that people will overinterpret the results. I was very careful to say that what this shows is that talin can activate a single integrin, and Ginsberg says that as well, but you might think that’s how it does happen in cells, but it might be that you require clustering in a cellular environment to bring together the right local concentration of talin in order to get the primary event, and maybe, you know, if the integrins are not clustered then the local concentration of talin isn’t enough.
TS: How will this research help us understand the bigger picture?
MH: The potential of the method is the most powerful thing. People have been struggling to figure out [the next steps] in the assembly of the adhesion complex. In theory, this kind of reconstitution experiment can address those sorts of questions. That’s where this is going to change the field.
Martin Humphries’ lab at the Wellcome Trust Centre for Cell Matrix Research studies the role of integrins in adhesion.