Andrea Globa is a neuroscience PhD candidate studying under the supervision of Shernaz Bamji at the University of British Columbia. Her recent first author publication on cocaine addiction in mice published in Nature Neuroscience garnered much attention, including interviews with numerous international news outlets. We sat down with Andrea to discuss her research and what it means for future of cocaine addiction research.
Your study focuses on the role of proteins called cadherins, which are found at synapses. Why were you interested in studying these particular proteins?
People always imagine that synapses look like the example shown in textbooks, with these membranes that are hovering in space and not touching each other, but there are actually tons of different proteins that are present at pre- and postsynaptic membranes that hold these sites in tight apposition to each other. Some of these cell adhesion proteins are called cadherins, and these have been shown to be particularly important for plasticity at synapses. You have to increase the amount of cadherin and cadherin-cadherin interactions sites to strengthen synapses in a process called long term potentiation, and cadherins need to be internalized and removed from synapses to weaken those synaptic connections in a process called long term depression.
A lot of people have studied this mechanism in the hippocampus and shown that cadherins play an important role in hippocampal learning and memory. We were broadly interested in whether cadherins were also important in synaptic plasticity in other regions of the brain, and whether they might be important in other types of learning and memory. That’s where this study came from.
Once you decided to focus on learning and memory, how did you go about determining the importance of cadherins in these processes?
In the first phase of our study we were looking at what happens to cadherin localization in wild type animals. We wanted to see if there were changes in cadherin localization at synapses in a region of the brain called the Ventral Tegmental Area (VTA) after mice developed a preference for cocaine. We did this using a behavioural task called Conditioned Placed Preference (CPP) in which a group of mice was placed in a three chamber box where they received cocaine in only one of the chambers during the conditioning phase. During this process, they learned to associate the rewarding effects of the drug with that context, so when returned to the three chamber box, they would spend more time in the drug-paired chamber compared to a saline control group.
We collected VTA samples from these wild type mice, and we processed them to do immunoelectron microscopy. We discovered that after they developed this preference for cocaine, there was way more cadherin localized to the membrane. Furthermore, the mice that had the strongest preference for cocaine also had the most cadherin at their membrane. So there was a strong correlation.
In the second phase of our study, we wanted to know whether stabilizing cadherin at the membrane using a transgenic approach would have any effect on behaviour. We expected to see mice that would develop a strong preference for cocaine that couldn’t be extinguished. But we wound up seeing the opposite. These mice did not get as addicted to cocaine, and spent way less time in the conditioned chamber compared to their littermate controls.
This seems counterintuitive based on the results of the first part of the study.
It did to us as well, but we did some reading in the literature, and discovered that the type of plasticity that occurs at these synapses is different than the type that occurs in the hippocampus. Normally there are AMPA receptors and NMDA receptors at synapses. Most AMPA receptors are GluA1/2-containing, meaning that they’re calcium impermeable, and this is true in the VTA as well. But after exposure to cocaine, these GluA1/2 heteromers are removed from synapses, and replaced by GluA1 homomers, which allow calcium into the cell. It’s thought that this helps potentiate these synapses as it increases their sensitivity.
So since this turnover in AMPA receptors has to occur for plasticity to occur, we hypothesized that perhaps the increase in cadherin at the synapse was preventing the turnover by stabilizing the receptors. We tested this by immunoelectron microscopy, and showed that GluA2 receptors are removed and GluA1 homomers are inserted after conditioning in wild type mice. But in our mutant mice that already had maximum cadherin levels, there was no change in AMPA receptors. The switch was blocked.
Has anything like this previously been shown for humans? What implications could this have for us?
Genome Wide Analysis Studies (GWAS) of people that are prone to addiction compared to healthy individuals have shown a large number of mutations in synaptic proteins. These include cellular adhesion molecules like cadherins, as well as their binding partners, the catenins. Since these types of studies identify a such large number of mutations, it’s often hard to figure out if the mutations really have any functional relevance. Our study suggests that these GWAS studies are valid, and that there probably is some reason that changing cadherin stability or the amount of cadherin in these individual’s brains might have an effect on their propensity for drug addition.
Our results are also interesting because they show that plasticity, and the rules of plasticity, are not the same across the whole brain. This could be important for developing therapeutic preventions down the line. Presynaptic proteins like cadherins are important for things like learning and memory, which we don’t want to interfere with or impair, and if the rules on how these proteins work were consistent across the entire brain, we might have trouble interfering with plasticity and addiction while leaving plasticity and learning untouched. So this will hopefully allow us to find a way to target plasticity in this reward circuit, while not affecting learning and memory.
How do you think these results would extrapolate to alcoholism or other types of addictive behaviors?
A lot of previous work showing this type of plasticity in the VTA has looked at use of amphetamines, alcohol, and nicotine, and found that some of these changes in the receptor type also occur. So the rules of plasticity probably apply to those situations as well. That could be something to look at more specifically.
Does your lab have any plans to follow up on this research?
Definitely! Cadherins are expressed throughout the brain, so it would be really hard to target them for therapeutic intervention. But if we were able to target another enzyme that acts on cadherins or one of their binding partners, we might be able to more specifically affect plasticity in these reward circuits while not affecting other brain regions. That’s something that our lab is interested in following up on.
I also think it might be interesting to look at what’s happening to cadherins and plasticity in the nucleus accumbens. The nucleus accumbens is one of the downstream targets of the VTA that is strongly implicated in addiction, and there are plastic changes that occur in response to drugs of abuse in this region.
You’ve had an incredibly successful time during your graduate studies! Do you have any advice for grad students going into your field?
Check out the lab that you’re interested in before you start, and make sure that you’re happy with the environment. It’s hard to be happy in a lab where you don’t quite fit in. In fact, I think the environment is more important than the project you choose. A lot of people get really hung up on the topic of their research. I’m not saying that’s not important, but your project can change very quickly in a few experiments, while the lab you choose will have much longer lasting consequences. It’s important to work with a supervisor who you get along with and who will support your career goals.
Thank you for your time, Andrea! It’s exciting to know that this incredible research is happening here in Vancouver!