So Sci was sitting in a seminar the other day. We were mentioning this paper, some problems we had with it, some of the things we LIKED about it, and various approaches, etc, in our usual sciencey fashion. As the discussion got intense (in a good way), one of the PIs there leaned over and said to Sci "you know, you should TOTALLY blog about this". Sci paused, and said "well...it's probably a little complicated for my readers".
And it is. The title alone. "Dnmt3a regulated emotional behavior and spine plasticity in the nucleus accumbens"?!?! This is complex stuff, much more complex and involved than I usually cover on the blog. It's got DNA methylation, how that affects things like protein transcription, how that affects BEHAVIOR, and what the heck does all that even have to do with SPINE DENSITY!??! It's a lot. It's complicated to understand, because it involves a LOT of background understanding, and it's complicated to explain, for similar reasons.
But then I thought about it. You know, my readers (all two of you) are some smart people. Many ARE in science, if in different fields, and many are interested in science. You guys can get this just fine, if I can EXPLAIN it. And it's an interesting paper, partially because of what it studies, and partially because of what it REVEALS about what scientists think about "emotional behavior" right now.
So. We're going to break it down. We're going to go through the hefty stuff behind this paper, one chunk at a time. We're going to cover the findings, we're going to cover the ideas, and we're going to cover the problems that the other scientists at the table had with this paper. We're going to go into why the problems with this paper exist, and what can be done about them, and we'll talk about what this means for the field of drug abuse and "emotional" behavior in general. We're going to do it over the next few days. Because y'all are PLENTY smart enough to get this, if you only have the background. And gaining expertise in a field is a lot about being able to know your background, where you came from, interpret it correctly, and move it on to the FUTURE.
Let's do this thing.
So let's start with that acronym, Dnmt3a, what it means, what that REALLY means, and why they are interested in it.
Dnmt3a stands for DNA methyltransferase 3a. And to get to what THAT means, we will have to dig deeper.
Welcome to DNA METHYLATION!!!
So. You know you have DNA.
DNA, as you know, makes up genes, which are specific sequences which code for a protein output (someday I will cover how this happens, as it is very cool, but today is not that day). So in order to get a specific protein output, you need to have that gene in your DNA, and that gene also needs to be EXPRESSED. Getting a gene expressed is a long and complicated process, but what we're focused on right now is not the process, but the beginning. Because in order for DNA to be expressed, it has to be AVAILABLE. This means that it can't be tightly wound around histones, for one thing. But even when it's not wrapped around histones, available DNA is not always available.
Let's look at why this is.
Up there at the top of the picture, you can see the base pairs as they make up DNA. What is blown up at the bottom is one of those specific base pairs, cytosine. What DNA methylation does is put a methyl group (highlighted in yellow) onto the cytosine.
It doesn't look like much, but this methyl group is very important. Depending on where it is located, it can either make expression of that gene go DOWN, or it can make it go UP. Too much methylation in some sites can result in cancer, but too much methylation in other sites can silence a gene entirely. Methylation has lots of important functions, including regulation of cell types, cell cycles, and all the way up to things like memory.
So that's basic DNA methylation, what about DNA methyltransferase? DNA methyltranferase (Dnmt) is an enzyme that causes methylation of DNA. There are various types, but the one we are interested in right now is Dnmt3a. And for this paper, what we are interested in is the role that Dnmt3a plays in addiction.
You might say: whoa, we're going from super tiny things changing DNA to addiction, which is whole brain and HUGE! Well, yeah. Little things have big effects. And while histone deacetylation and methylation is today, DNA methylation is forever (or at least for a really long time). Because addiction is such a long-lasting, relapsing disorder, it stands to reason that some semi-permanent to permanent changes are being made. We know that Dnmt3a is expressed in brain areas like the nucleus accumbens, an area that is very closely associated with the rewarding and reinforcing properties of drugs like cocaine.
So to look at whether the DNA methylation could be changed in response to drugs like cocaine, the authors of this study looked for the various DNA methyltransferases in mice that had either received a single injection of cocaine.
And here's what they got. What they got is, at first, not quite what I expected, but when you think about it, it makes sense. Immediately after the cocaine injection, there was a SPIKE in Dnmt3a, but after about 24 hours, the levels of Dnmt3a were decreased. If you've got more Dnmt3a being made (which is what this measured), you presumably have more DNA methylation taking place. So it looks like with one injection of cocaine, you get an brief increase in DNA methylation, that drops off pretty soon after.
What confused me at first was why the levels didn't STAY high, why they dropped off. But this makes perfect sense, really, you don't want whatever changes are happening in response to cocaine to keep happening, and you certainly wouldn't want to go overboard on DNA methylation. So the methylation drops off.
Dnmt3a is not the only Dnmt involved here, but it was the only one that showed significant changes in response to cocaine.
But that's only one injection. What about lots of cocaine over a long period of time?
Here you can see on the left, mice treated with cocaine for 28 days, and THEN give 28 days withdrawal, and on the right, rats self-administering cocaine (taking their own injections their-ownselves), and then given 28 days withdrawal. In both cases, they looked at three different dnmts, and only Dnmt3a was increased, which means that while it spikes once in response to ONE injection, chronic cocaine (given either by injection or self-administered) makes it stay up for a while. That's a lot of DNA methylation going on there.
And now, the critique thus far: So far (the first two figures), we're ok. It's PCR, and you can't really argue too much with PCR. My only critique (and I'll get to this further into the paper as well), is this. Mouse, mouse...rat? why did they do all this PCR in the mouse, and then switch to rat? Or rather, why did they do the RAT self-admin, and then do mouse for everything else? It's much harder to get mice to do coke self-administration, so I don't blame them for using rats. But PCR? You can do PCR on a rat brain just as easily as on a mouse brain. So why the switching back and forth?
So so far, we're through the first two figures, and we can see that DNA methylation may change with just ONE injection of cocaine!! Tune in next time to find out what this does to...reward.
LaPlant Q, Vialou V, Covington HE 3rd, Dumitriu D, Feng J, Warren BL, Maze I, Dietz DM, Watts EL, Iñiguez SD, Koo JW, Mouzon E, Renthal W, Hollis F, Wang H, Noonan MA, Ren Y, Eisch AJ, Bolaños CA, Kabbaj M, Xiao G, Neve RL, Hurd YL, Oosting RS, Fan G, Morrison JH, & Nestler EJ (2010). Dnmt3a regulates emotional behavior and spine plasticity in the nucleus accumbens. Nature neuroscience, 13 (9), 1137-43 PMID: 20729844