PI3K-Like Protein Kinases

What is the impact of TOR on nuclear dynamics?

Dr Susan Gasser - Friedrich Meischer Institute, Basel, Switzerland

From the very beginning I’ve been interested in the 3D organisation, so the spatial organisation of cellular functions. This was a bit unusual thirty years ago, now it’s much more common that we realised that things, enzymatic pathways or control pathways, are not only a matter of sequential interactions or sequential enzyme activities but also their spatial organisation, both within the nucleus and between the nucleus and the cytoplasm. So my approach was largely through microscopy to begin with and now there are many other methods to analyse the juxtaposition of molecules in the cell. I think this is important because in cancer, in many diseases, what you see is a morphological change in the tissue or in the cells. If we look in sub-cellular levels we also see a morphological change of cytoskeleton, of nuclear organisation, of the spatial arrangement of chromosomes in the nucleus. So I’ve always aimed to try to ask this question: how does the spatial organisation reflect or determine normal function?

What is the impact of TOR on nuclear dynamics?

That’s quite interesting. TOR actually is a cytoplasmic enzyme that controls on the one hand protein synthesis and on the other hand the TOR2 complex, the actin cytoskeleton. So at the beginning we didn’t think TOR would have anything to do with the nucleus but we did a screen, found an inhibitor that was specific for TOR2. There are two mTOR complexes, TOR1 and TOR2, and the TOR2 complex regulates the polymerisation of actin. So what we learned was that actually if you inhibit actin polymerisation you accumulate G-actin in the cytoskeleton. This is actually taken up into the nucleus and it perturbs pathways of repair. So there are many nuclear enzymes that depend on actin, chromatin remodellers depend on actin in the nucleus in the complex but there’s also a pathological effect of an excess of globular actin in the nucleus.

So very indirectly inhibition of TOR2 in the cytoskeleton impacts normal cytoskeleton turnover, elongation, maintenance of actin filaments. The perturbation of this generates G-actin, a pathological amount of G-actin, which then in turn impairs DNA repair.

Are you looking at different phases of the cell cycle?

Actually we’re looking at it mostly in a G1/S phase. It doesn’t seem to be, contrary to many of the damage effects that we study which are linked to replication, this perturbation of repair through actin seems to be cell cycle independent. So the pathway that it interferes with is not replicative repair it is basic cision repair. Basic cision repair takes place throughout the whole cell cycle, it’s one of the workhorse repair mechanisms that’s going on all the time at a high level to take care of modified bases, oxidised bases, abasic sites and so forth.

So, again, if we want to relate this to cancer or to clinical situations we know that in cancer cells mutations in Ras or Rho, Rac, many things that perturb, actually, cytoskeleton organisation are transforming. Most people have thought this has something to do maybe with signalling, signalling through kinases and it does, in part, through MEK kinases or MAP kinases. But there’s a second pathway where it could be signalling through the lack of polymerised actin. So we see cancer cells round up, change their form or sometimes become even more invasive, more aggressive. These changes in cell shape clearly correlate with changes in cytoskeleton and I would say that also we haven’t proven that link yet but I think that may actually impact genome stability.

What do you think the outcomes of the meeting will be?

It’s very exciting. Again, it’s a bit like the effect of TOR inhibitors that I just described. I actually come from the nuclear point of view, I study DNA repair mostly, and by chance a screen brought me to a site, what I always call the quintessential cytoplasmic kinase, which is TOR. So I think the field is at a point now where we see that everything talks to everything in some way and what we’ve heard are talks which describe how metabolism and changes in metabolic pathways affect DNA repair. We’ve heard about how vacuole function is implicated in genome integrity. So I think we are at a point in the field where we start to have to think about the cell as a complete system. We’re no longer studying a certain type of DNA damage in vitro or with purified enzymes, that goes on, but to understand phenomena like cellular transformation or even control mechanisms over types of repair we have to integrate cytoplasm and nucleus, see how the cell as a whole is responding to its environment, reflecting its differentiation state. This conference is just at the forefront of that. Some of these people don’t usually meet and that’s always very important, to bring together people who don’t talk every weekend.