Sex Differences in the Neural Control of Thermogenesis: An Interview with Dr. Stephanie Correa

Understanding the complexity of the human mind and body is a feat that some would say is impossible. While the scientific community forges on despite the immense expanse of the unknown, one thing is certain: understanding the complexity of human biology is absolutely impossible if we only study half of the population. For years, clinical trials and experiments were limited to strictly male participants and male model organisms. The scientific world failed to acknowledge the role of sex in many biological functions and higher order processes besides reproduction.

Fortunately, researchers now better understand how sex influences human biology, including response to diseases and their therapies. This new awareness has created a community of researchers whose mission is to understand the biological differences between males and females and leverage these differences to create more effective disease interventions. Among these pioneering scientists is neurobiologist Stephanie Correa. Dr. Correa received her Ph.D. from Cornell University in Neurobiology and Behavior and conducted post-doctoral work at the University of California San Francisco (UCSF) studying neuronal regulation of physical activity and body weight in female mice. Dr. Correa is now a part of the Brain Research Institute (BRI) at the University of California Los Angeles (UCLA) where she runs a neuroendocrinology lab researching how sex hormones like estrogen regulate temperature homeostasis and metabolic health.

To help elucidate some of her contributions to the burgeoning field and to help shed light on the amazing research being conducted at the UCLA BRI, Knowing Neurons interviewed Dr. Correa on her recent publication in Nature Metabolism and on her other ongoing projects in the lab.

What would you say is the main goal of your research or the main questions you are pursuing in your lab?

The main questions that we’re interested in are related to how estrogens affect metabolism. We focus on the brain because I think it’s really interesting to think about estrogens coming back from the periphery—from the ovaries and other tissues—in both sexes and affecting so many aspects of physiology. When I started, a lot more was known about how reproductive hormones affect reproduction, but I thought it would be cool to try to understand how reproductive hormones affect other things like metabolism. Our goals are twofold: We want to know what cells are involved in regulating metabolism in response to estrogen, and also how estrogens affect those cells. We started to make progress on the identity of the cells, but the next step will be to figure out how estrogens then change the activity of those cells.

What inspired you to pursue the field of hormone and metabolism research?

Initially, when I found out that estrogens regulate feeding, energy expenditure, and body weight, I was surprised. I thought to myself, “Why would they do that? Estrogens are for reproduction.” I was thinking in a very limited way, but of course, it made sense after thinking about it. In mammalian females, energy and reproduction are tightly linked. It is really important to have the capacity to regulate energy expenditure in order to be able to reproduce. I just thought it was super interesting to drill into that and figure out how that happens. Actually, I wasn’t super into the brain before that question came to my mind, but we need to understand how it is all regulated centrally.

Why is it important to understand sex-based differences in metabolism when considering therapeutic interventions?

I came from studying reproductive hormones in female birds during my Ph.D., and I decided to study mice because I wanted to use genetic approaches. Back in those days, genetic knockout was only available in mice and flies. When I started learning about maintaining mouse colonies, I was really confused. Everyone was only studying males, and it is significantly easier to maintain females. They don’t need to be housed separately, and there was way more flexibility, so the focus on males didn’t make sense to me. Now, I think it is important to counter the bias that resulted in a focus on males. Why would we only study one half of the population? Why, if we are interested in understanding human health and disease, would we focus on only half of the problem. There is a lot of evidence that men and women can respond differently to the same treatments or have different risk levels for certain diseases. I do not think we can understand physiology, health, or disease until we understand it in the whole population.

In your recent Nature Metabolism paper, you explore the role of estrogen receptor alpha in energy expenditure. What was the specific question you were attempting to answer, and what led you to pursue this question?

As a postdoc, I found a population of neurons in the hypothalamus of mice that regulates physical activity, but not reproduction and not thermogenesis. The hypothalamus does many things. It regulates different aspects of physiology, specifically reproduction, behavior, and metabolism, but the population of neurons I had found using a mouse knock out was very selective—really specific in its function. When we got rid of it [the population of neurons], none of these other things [reproduction, metabolism, etc.] were affected, only physical activity. So that suggested that the hypothalamus might have other populations of neurons within it that are also specialized and dedicated to regulating its other functions such as reproduction or thermogenesis. Using single-cell RNA sequencing we asked how many different populations exist in the hypothalamus. And from there, we hoped to find the population that controlled thermogenesis because we were interested in energy balance.

What were your key findings?

We found multiple cell populations in this part of the brain called the ventromedial hypothalamus, and Laura Kammel, the Ph.D. student who was working on following up and validating the results of the single-cell RNA sequencing, did in-situ hybridization to find the top marker of each of the populations and find out where the populations are. When she looked at different populations, she expected to find different spatial patterns of gene expression indicating neuronal population location. Surprisingly, she only found two populations in the region where the estrogen receptor was located. So that made things easier for us because we were only interested in populations that were estrogen-responsive in female mice. One of the two populations was one I had already characterized to be important in physical activity, but in the other population was labeled by a gene called reprimo that nobody had ever found in this part of the brain. There was evidence in the literature that it was estrogen-responsive, and when Laura did in-situ hybridization, she found this gene was expressed in this population of cells in females. That was really exciting because we already knew that energy expenditure is only regulated in response to estrogen signaling in this region in females and not in males. If we are looking for the gene that regulates energy expenditure in response to estrogen signaling, then it would make sense that it is a gene that is only expressed in females. So, then we wondered what this sex-specific estrogen-responsive gene did. So, Laura knocked it down and saw an effect on thermogenesis and temperature.

Which, if any, of these findings were most surprising and why?

It is interesting because we found these two populations in the estrogen-sensitive region of the nucleus. We asked what the top differentially expressed gene was in that population and it was reprimo. We then had to validate the functional relevance of reprimo because high gene expression doesn’t always correlate with high functional impact. We were excited when we knocked it down and saw an effect that confirmed its functional importance.

What are the future directions for this project? Is there translational potential for your results?

It is really cool that we were successful in identifying a population of neurons responsible for estrogen regulation of thermogenesis and that we defined it with a gene, but now we’re moving toward figuring out the second goal: what are the cells, the reprimo expressing cells, and how does estrogen regulate these cells? Preliminary data suggest that estrogen might regulate reprimo expression. So that is what we’re going to move towards testing. The next questions are: is reprimo altered by different estrogen levels, and is reprimo required for estrogen effects on temperature?

In this study, you use tools like single-cell RNA sequencing. How have high-throughput sequencing tools such as this transformed the field and aided you in your research?

At first, I was reluctant to get into single-cell RNA sequencing because I didn’t want to just jump on every fad. There is definitely a single-cell trend in science right now. I thought it was an appropriate tool for the question to actually determine if this region of the hypothalamus was heterogeneous—if it had different types of cells and neuron populations within it—and transcriptional profiling from the RNA sequencing told us that there was heterogeneity in this region of the brain. In this case, I believe it was appropriate; I didn’t want to use it just to use it. Maybe the less popular approach of just knocking the gene down is not as cutting edge as single-cell RNA sequencing, but I thought it was actually quite powerful too—that we were capable of manipulating the gene that we found. So, I really liked combining the single-cell RNA sequencing with the functional test.

Are there any downsides to these high throughput techniques?

For us, there was a downside. We were able to find different cell populations, but how many different populations one is able to detect depends on the parameters of the analysis. The scientist who actually did the analysis, Ed van Veen, settled on something that seemed stable and robust. The thing that the RNA sequencing could not tell us was anything about sex differences. We did not have the power with the single-cell RNA sequencing to compare the males and females. We did not see any sex differences when we just looked at the bioinformatics. It was not until Laura looked at the in-situ hybridization data that she saw this outstanding sex difference between males and females in reprimo expression. If we had just stopped at the bioinformatics, we would have missed a lot of the important biology. So, the independent and more sensitive validation is really important.

Are there any other major ongoing projects in your lab right now?

This Nature Metabolism paper was the first paper that we published on our own as a starting lab, but we have another story that is being led by my postdoc, Zhi Zhang. He found estrogen-sensitive neurons that regulate torpor. When he activated estrogen-sensitive neurons in another part of the hypothalamus called the medial preoptic area, he was able to induce torpor. That was a surprise because we were manipulating estrogen-sensitive neurons in a brain region thought to regulate temperature, so we thought we would see estrogens regulating temperature regulation. However, what he discovered instead was something a lot more basic. Torpor is a basic function of the brain that initiates a particular metabolic state that animals go into when energy is scarce. So that was something we stumbled on that was very exciting, but we ended up finding very few sex differences. However, it was cool that these estrogen-sensitive neurons had very similar roles in males and females.

Throughout your years as a researcher, what have been some of your most exciting discoveries and why?

Sometimes the findings that are most exciting for me are not the super flashy or impactful ones. I get really excited whenever I find something surprising or unexpected. I think it’s really fun when you’re expecting one thing and the data tell you something else. It’s fun to be open to something different, something new. An example of that is our paper last year where I made a knockout mouse for an estrogen receptor in the hypothalamus, and I expected the mice to gain a bunch of weight, but they didn’t. Other publications were informing this hypothesis, so the data was not in line with the literature. However, there was a tiny increase in body weight, so I asked what led to that? I did an analysis that looked at bone, and it turned out the bone density was very unusually high in these mice. I didn’t see the effect on feeding and body weight that I was expecting, but suddenly I saw estrogen-sensitive neurons in the hypothalamus regulating bone density! So that paper was left to the lab I was initially in, and they did more work looking at specifically which neurons in the hypothalamus regulate bone density. They will figure out the mechanism, which will be new and important, but thought it was fun to find something totally unexpected.

What part of your job as a researcher do you enjoy most, and which part do you enjoy the least?

There are so many aspects of the job that are not related to science, but I have been surprised by how much I like some of those aspects. For example, I found that I really enjoy mentoring students. It turns out I love it when my students come up with a really good idea or ask a really good question. That is not something that I trained for, but it is something I very much enjoy!

What advice do you have for young researchers interested in pursuing graduate school or a career in research?

You don’t do this job for the money, the fame, or the accolades. You do it because it’s fun, and because it’s fun to be the first person to discover something new—to solve the puzzle. When you have a surprising finding, you then naturally ask, “Well, how does it work?” Just figuring things out is the really fun part, and that is my motivation. So, my advice is to find what drives you and pursue that, whether it’s research, science writing, science policy, etc.

Written by Arielle Hogan. Photo Credit: Todd Cheney, ASUCLA Photography
Edited by Elizabeth Burnette and Sean Noah.

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