Reader David sent me this paper the other day, and asked if I could blog about it. I said ok, maybe, and then I read...
Sounds very cool, doesn't it? Sounds like the FUTURE! Where's my JETPACK!!!?!?!
But of course "gene therapy" is kind of a buzzword. A lot of people throw it around, but it seems like a lot of people don't know what it really MEANS, and what it can be used for.
But it turns out, it can be used for quite a lot! And it may not be quite so far in the future. After all, they're marketing jetpacks.
So let's start with gene therapy and what it is, and then we'll go into why they used it in this particular paper. Gene therapy is based on the idea of inserting a gene into someone's genome, either in the whole body or in specific parts, to change the gene expression of that cell or group of cells, and to use this technology to treat disease. In this case, what we're talking about is viral-mediated gene expression. This is where we use a virus (for our own nefarious purposes mwah-ha-ha-ha!!), take out the nasty bits of the viral DNA, and load the virus with the gene you want to express. You then inject the virus into your area of interest (normally this is really site specific), and the virus, using its own virusy ways, will insert your gene of interest into your area of interest. The gene will get incorporated into the genome, and get expressed by your cells!
This is a really useful technique that is now being widely used in a lot of fields, including neuroscience. You can insert genes to increase expression of a specific protein, OR use genes that DECREASE expression of that protein. So it allows you to carefully and selectively look at a specific thing in a specific brain region (in neuroscience, anyway).
Now we've got gene therapy, what about p11?
p11 is a relatively new protein on the depression scene (I think the original paper was only as early as 2006), but the effects got everyone excited. p11 is a protein that controls the expression of two serotonin receptors. Serotonin is thought to be one of the major players in mood disorders like depression (while selective serotonin reuptake inhibitors may not work in many patients, this doesn't mean that serotonin isn't playing a role, it's the role that it's playing that's up for debate), and so the consequences of p11 became suddenly interesting. And it turned out that mice with a knockout of the p11 gene displayed what we call a depressive phenotype, where they show more immobility (which used to be called behavioral despair, but now we don't think that's accurate) in tests such as the forced swim test and the tail suspension test, tests which are sensitive to the effects of antidepressants.
The thing is though, that these p11 knockout mice have p11 knocked out EVERYWHERE. That's some big changes, and they are also changes that exist during the whole of the mouse's life, and so could affect a lot of other things. In order to really narrow down what p11 is doing and where it is most important in depression, the authors of this paper wanted to take a normal mouse, and change p11, in very specific brain regions.
The two brain regions they picked were the nucleus accumbens (they abbreviate it NAcc, but I hate that, I prefer NAc, because to me NAcc means the NAc CORE, but whatever) and the anterior cingulate cortex. The nucleus accumbens is a brain area located here:
I blog about it a lot because it's thought to be involved in the rewarding and reinforcing properties of drugs like cocaine. But many people have thought that it could ALSO be involved in some of the issues associated with depression, specifically things like anhedonia, which is a lack of the ability to feel pleasure, and thus very closely related to things like reward.
The anterior cingulate cortex on the other hand, is located here:
They are interested in the anterior cingulate cortex because a HUMAN study found that depressed humans had lower levels of p11 in the anterior cingulate cortex.
So what did they find? Let's start with the NAc.
To start with, they used a virus to insert a gene into the NAc which knocked DOWN p11 in normal mice, only in the NAc. All of the pretty glowy pictures there are of them proving that they can do that. What we are interested in is the three graphs at the bottom (the bar graphs), which all show immobility. These are immobility measures in the tail suspension test and the forced swim test. The first two (left and middle) show that mice with a knockdown of p11 only in the NAc show increases in immobility in these tests, which indicate a pro-depressive phenotype. The graph on the right shows treatment with imipramine, an antidepressant, and shows that the antidepressant still exerts some effects when p11 is knocked out. So it looks like, so far, that these mice are more "depressed", but that the still respond to antidepressants.
So then the question is, can ADDING p11 rescue a "depressed" mouse? To do this, they used p11 knockout mice, that had had p11 knocked out all their lives, and this time used the viral vector to INCREASE p11 in the NAc. When they increased p11, they got a reversal of the "depressive" effects seen in p11 knockout mice. These mice showed more swimming in the forced swim test, more struggling in the tail suspension test, and even showed increased sucrose preference (which is through to be a measure of anhedonia). You can see all that here:
The diagonal bars show the animals that had p11 increased. The sets of bars on the right are the p11 knockouts, and you can see they show longer immobility, until the p11 is increased in the diagonal bars.
But of course, all this stuff was in mice. How does this relate to HUMANS?
They looked at human post-mortem tissue (that's dead) from depressed and control patients, and they tested each NAc for p11.
Looks nice, yeah?! The depressed patients showed a DECREASE in p11 in the NAc (which is comparable in the mice to knocking down p11 in that area, as shown in the first set of figures).
They WERE going to look at the anterior cingulate, but it all got shoved to the supplemental data because there was no difference (and I had to hunt around for that little sentence. Stupid supplemental data!). But that's ok.
So this all lines up, and it all looks pretty nice. Sci likes this paper, it's neatly done, and they nicely knock something down specifically, increase it specifically, and look at behavior as a consequence. Good and thorough. But the question is: now what? p11 controls the expression of two serotonin receptors (the 5-HT1B and the 5-HT4). Which of these receptors is undergoing the changes from lack of p11 that is causing the depressive phenotype? Is it both? Are they expressed in the NAc? Or do changes in p11 changes serotonin receptor expression elsewhere? Other brain areas like the hippocampus have been implicated in depression, and also in the actions of antidepressants, does p11 play a role there as well?
And once we've got this worked out, what are we going to do about it? It's too much to ask to give a viral vector in the brain of every depressed person. In fact, we can't really do that yet at all, it's a very risky technology still (especially in brain). Are there drugs that increase p11? What about the specific receptors, the 5-HT1B and 5-HT4? 5-HT1B and 5-HT4 receptor activating drugs have been shown to be effective in animal antidepressant tests, but have never been developed. Should we develop those? Or go after p11? And of course, although there are studies showing p11 is lower in depressed patients, there are also studies out there showing that p11 isn't really changed in depression, after all.
Ah, science, always with the more questions! But you keep an eye on p11, this little protein looks like it might have some potential.
Alexander B, Warner-Schmidt J, Eriksson T, Tamminga C, Arango-Llievano M, Ghose S, Vernov M, Stavarche M, Musatov S, Flajolet M, Svenningsson P, Greengard P, & Kaplitt MG (2010). Reversal of Depressed Behaviors in Mice by p11 Gene Therapy in the Nucleus Accumbens. Science translational medicine, 2 (54) PMID: 20962330