Session discussion
Chair: Professor Julie Campbell, FAA
Speakers: Professor Bob Graham, Professor Brandon Wainwright, Dr Melissa Little and Professor Perry Bartlett

Chair – We can now direct questions to all four speakers in this session.

Question – This question really relates to what Perry has been talking about, but also to the other tissues that other people have talked about.

In the brain it is now well known that there is a tight metabolic link between astrocytes and neurons. It is also obvious that the brain volume is constant. So something has to give when the number of neurons increases. The number of astrocytes, I presume, has to decrease. What is actually calling the shots – is it the astrocytes or the neurons?

Perry Bartlett – Well, that question is bigger than you and me. On the idea about maintenance of cell number and size you are right, although it is an interesting thing: the olfactory actually does increase in size, at least in the mouse, as it grows older. As to the hippocampus, it is not clear whether it is continuing to grow. But in general you are right, there is as much cell death going on as there is replacement.

In fact, a beautiful experiment has been done à la bone marrow replacement; that is, there needs to be a niche for it to get into. Jeff Macklis has done an absolutely heroic experiment where he back-fills neurons – which will retroactively transport anything from their target back to their cell body – with photo-activated toxins. He can irradiate specific populations of neurons in the cortex, either layer 5 or layer 6, corticothalamic or corticospinal, and he has shown that when you do that, those cells that normally migrate to the olfactory bulb actually migrate to the cortical areas and replace the corticothalamic.

Now, in results that may be still unpublished or may have just come out, he has also shown that they will replace the corticospinal. This is really quite mind-blowing in an adult animal. Not only do they replace the corticospinal, but they put out processes that go through the mid hindbrain into the spinal cord. So the ability to replace nerve cells seems to be unlimited in that regard. There are no great barriers.

I can’t really answer the neuronal glia part of your question, because who knows what the dominant partner is there. I mean, in a lot of diseases, like motor neurone disease, it is becoming more and more found that the astrocyte may be the sick cell and it is the neuron that consequentially dies.

But in the answer to the growth part of your question, I think in some areas you are increasing volume and in other areas it is steady-state.

Question – This is a comment, really, on something Bob Graham said, and then related on to something Perry said.

Bob, you commented that you can only use the sickest patients in trials of experimental therapy. Ethically, there is every argument for refuting that. That is, ethically it is the balance between potential benefit and potential harm that is important, and sometimes the potential benefit outweighs the potential harm much more in people who are not sick at all.

The example I would give is related to what Perry talked about in Tony Hannan’s work on Huntington’s disease. I don’t know if you know, Perry, but four years ago when that work was first published, when the first data came from Oxford, I suggested in Australian Medicine that we should do neonatal testing to see which babies might develop Huntington’s disease so that we could provide an enriched environment for them. This was treated, I may say, not only with scepticism but with derision by our clinical genetics colleagues, who seemed to think this was an outrageous idea.

I actually still think it is a good idea. I think that if we have a potential therapy that can prevent a disease, that is far and away the best way to go, in ethical terms. And I should say, Brandon, that the same is true for cystic fibrosis. One of the problems in the London trial which Brandon and I were both involved in was that we were using people in end-stage disease, who are the hardest people of all to treat. I don’t think gene therapy would have worked, even if we had used babies, but nonetheless the people we were using biased it against success.

I just think we should get away from the concept that we have to use people who are on the edge of death in order to try an experimental therapy. I think we should really be thinking in terms of using therapy for prevention as much as for treatment.

Would any of the panel like to comment?

Bob Graham – Can you come and sit on our ethics committee? I think your point is well taken, but practically it is just a lot easier to start with the sickest group of patients.

I think the other point that should be made is that even the small benefits that we have seen so far in the trials are very nice to see, because we are obviously treating a very sick group of patients and one would hope that in the lesser-ill patient you might see greater benefits. But your point is well taken.

Perry Bartlett – I totally agree with Bob. I guess we were all incredibly sceptical of environmental influences on things – certainly I was – until some of this data started to turn up. The dentate gyrus weight in those animals is just incredible, the fact that it is maintained as it is and that they perform in spatial tests the same.

It may be true of motor neurone disease; there are some indications coming out now regarding SOD1 animals, with the gene that is mutated in familial ALS/MND, given the motor improvement that is shown here. But we don’t actually know what the motor improvement was. It is probably more synaptic-related than it is neurogenic-related; nevertheless, we don’t know that.

Many of these familial degenerative diseases may benefit from this sort of environmental enrichment, whatever that means. What we don’t know is what it does mean. Where do you start in terms of those sorts of paradigms? You don’t get people crawling through little black tubes – or perhaps you do, Bob, I don’t know!

Question – I do ethics, and I am interested in human enhancement. My question is to Perry Bartlett. Can these mechanisms be employed in normal animals to improve function on the basis of neuronal proliferation?

Perry Bartlett – You can show increases. Again I think we are limited by the sensitivity of many of the behavioural tests. So it is not until you get quite big Deltas that you start seeing real differences. That’s why the Huntington’s model is such an attractive one. It is not that we are so interested in Huntington’s disease, it is just that there is such a big Delta and such a rapid Delta between the sick animal and the normal animal.

There is no doubt you can show increased neurogenesis, as I showed you in that first slide of enrichment, and more cells being rescued in the hippocampus in a normal animal. Now, as to how that relates to behaviour – there are indications that those animals are smarter, but that Delta is not very big.

The experiment we are trying to do at the moment is to construct animals where we can specifically deplete those neurons as they are being made and being integrated, to see what effect that has in a behavioural sense. Although that is a negative indicator, I think it is going to give us a much clearer understanding of what the importance is.

I guess my hypothesis is really that this is a means of adapting your repertoire of responsiveness to environmental input – which doesn’t sound all that crazy, does it? In fact it sounds pretty obvious, I think. Certainly it appears to be happening within the olfactory system. We do appear to be selecting cells on the basis of stimulation of new olfactants, and it appears that those interneurons that are migrating in, which modify how you perceive smell in the cortex, are being turned over quite rapidly and it is very sensitive to physiological input.

What I am trying to say is that it really is an important mechanism, but to look at the importance one really needs to rack up the Delta so you can see how important it is to maintain that in terms of behaviour or in disease.

Bob Graham – Perry, could I just ask something. If I remember correctly, there was a nice paper showing that one of the reasons that malignant cells, when they metastasise, can escape anoikis, which a process of spontaneous apoptosis, is up-regulation of p75 neurotrophic factor receptor. Have you looked at the apoptopic rate in your cells? Maybe that is one of the explanations for why they are able to migrate and survive.

Perry Bartlett – It is an interesting thing, because Lizzie Coulson, in the lab, has got a group working on signalling through p75 in terms of cell death. And, interestingly – I didn’t have time to show you the data – there doesn’t seem to be the cognate track expressed in the stem cells. Normally that would result in cell killing, when you only have p75 alone. But there is something different about the stem cells, in that it doesn’t seem to be involved in that.

I am certainly aware of the p75 signalling in tumour cells to cause cell death. One of the things that Brandon says, and that we are very interested in, is that these stem cells that we are looking at here are probably the origin of gliomas, which are also incredibly difficult to treat. And finding that needle in the haystack, which is that slow-cycling or drug-resistant stem cell, is very important.