Big questions about the smallest heart muscles

May 22, 2024

Dr. Angie Greenman shares how she flexes muscles in the lab, studying the physiology of protein that could hold clues for a deadly heart disease.

Science Talks Podcast Episode 55 Big questions about the smallest heart muscles Dr. Angie Greenman

Understanding the role of cardiac myosin binding protein-c in muscle function can offer insight into cardiovascular disease. Mutations and abnormalities in this protein can lead to defects in the cardiac system, including heart failure in adults.   

Amy Barber and Caroline Bartelme are joined by Dr. Angie Greenman, a 2024 BIO5 Postdoctoral Fellow in Dr. Samantha (Sam) Harris’ lab in the Department of Physiology at the University of Arizona College of Medicine – Tucson. Dr. Greenman studies the molecular mechanisms of cardiac myosin binding protein-c that will hopefully lead to better outcomes for understanding muscle function and cardiac disease. 

This interview had been edited for length and clarity.

We always like to get started with rapid fire style type questions. What do you prefer, coffee or tea?  

Coffee. And then of course, tell us what your favorite coffee drink is. I lived in New Zealand for a while, and I got really into flat whites. 


What is the most adventurous thing you've ever done? 

Such a funny question, because I feel like I'm not very adventurous. I'm afraid of heights, but one time my friends convinced me to jump off a cliff into this beautiful waterfall. I felt that I had a big adrenaline surge after that! 


What is the best piece of advice you've ever received? 

In my life as a postdoc, it’s to not rely on just one mentor, but reaching out to other people and build your network. Not one person can give you all you need in terms of mentorship. 


We're always curious as to our guests’ backgrounds. Can you tell us how you became interested in science? 

My mom was a pharmacist. I feel like I was always exposed to health sciences. For a while I wanted to go work at a clinic, so I did the biology and premed route. Then I thought I should join a lab, because that'll look good on a medical school application, right? 

So, I joined a lab looking at skeletal muscle function, and I fell in love with it. At the time, I thought people who got their PhDs were insanely smart weirdos. And there's some of us like that, but that experience made me see science as a feasible career goal for myself.  


You were in New Zealand for your PhD. Can you tell us about that? 

I was doing my master's at the University of Wisconsin, and as I was finishing up writing my thesis, my mentor was in New Zealand starting a new collaboration. And instead of responding to my emails about questions and concerns around my CV, he asked if I wanted to move to New Zealand because they needed someone like me to do a PhD in that lab.  

I thought that sounded amazing, moving abroad was a great opportunity and it would be my first time. You can tell from my accent that I'm American, and even though they speak English, it's a different dialect. There are also different cultural norms, so I think a real strength of living abroad during my PhD was a better emphasis on work-life balance.  


What was it that brought you to the University of Arizona and to BIO5? 

I knew I wanted to do a postdoc and I'm interested in getting a tenure track position. I want to do a combination of teaching and research down the line. For those who don’t know, you need extra training in-between a PhD and an assistant professor.  

So, it came down to what kind of science I wanted to do, and I knew I wanted to stick with measuring muscle function. That narrowed it down for me. Plus, I knew I wanted a mentor who was a woman in science. I've had great mentors who were men, but I wanted to diversify my mentorship team. Dr. Sam Harris' lab fit all those needs, and I liked Tucson as well. I reached out to her, and it worked out. When I say it out loud, it sounds simple! 


Can you tell us a little more about your current lab and mentor Samantha Harris? 

Dr. Harris is focused on this one protein in muscle. It's in skeletal muscle and heart muscle, but we primarily focus on the heart.  

For a little background, the smallest functional unit of a muscle is called a sarcomere. There are different filaments that kind of overlap with each other, and these filaments are made up of proteins. One of those proteins is the protein we study in our lab, called cardiac myosin binding protein-C. These filaments are important because they slide past one another. Actin and myosin are mainly the two proteins doing this and they produce force.  

You know when you flex your bicep, you feel it gets stiffer. That's the myosin and actin interlocking together. What we find is when we get rid of this cardiac myosin binding protein-C, there's a lot of dysfunction, and it goes down to that sarcomere level. 


How do the Harris lab goals align with your career goals? 

I was already using similar tools to measure muscle function, but the Harris lab had a few more protocols that I hadn't used before.   

I wanted to get more sophisticated at these measures and learn more molecular biology techniques. I purify a lot of the protein C protein, for short. That takes cloning skills, working with E. coli, and other things I hadn't done before in my PhD work.  

It’s fun and I feel like I'm kind of like a mad scientist in the lab, like creating new things. 


Tell us more about your current project.  

Cardiac myosin binding protein-C is a long protein that can interact with both actin and myosin. But when that happens, we don't understand. It seems to affect how fast contraction can happen, either in a positive or negative way. When it interacts with actin or myosin, it has a totally different effect on contraction.  

I'm studying the middle portion of this protein. And previously, people thought that all it did was link the two ends together. But it seems to interact with actin and myosin directly, or indirectly, affecting muscle functions.  

In hypertrophic cardiomyopathy, that can lead to heart failure because some patients have mutations within this region of the protein. And we don't fully understand how that mutation links to their symptoms.  

I'm trying to better understand the normal physiology of this protein to focus on those specific mutants we see in the clinic. 


Was the middle section of the protein not studied before? Or how did you discover there was more going on with cardiac myosin binding protein-C? 

It was studied a bit, but it was difficult to study with the models that we have. What was cool about joining Dr. Harris's lab was that she had a model where we can literally cut and paste different segments of the protein. And doing that with certain deletions, substitutions, or mutations, that can tell us which parts of the protein are important for different aspects of muscle function.  

Before we had that model, it was too difficult because you had to look at tiny segments of the protein. But that doesn't always translate to what's happening in your heart right now. This model now provides us the ability to test these questions specifically.  


Can you break down and explain to us what exactly cardiac myosin binding protein-c is and why it is so important? 

It’s one of my favorite proteins now. For patients with hypertrophic cardiomyopathy, a lot of the mutations lead to the truncation of the protein, which essentially means that the protein isn't there, or only a segment of the protein is there. And when that happens, they have enlarged hearts, symptoms like shortness of breath, and an increased risk of sudden cardiac death.   

It’s just one protein, and you can survive without it, at least in your heart, but it can lead to a poor quality of life. 


What is hypertrophic cardiomyopathy, and how does that protein play a part in it? 

Hypertrophic cardiomyopathy is a genetic disease and it's only known to be within the sarcomere. When any of those sarcomere proteins, include myosin and cardiac myosin binding protein-C are the two most common proteins that get mutated. When these proteins are mutated, it seems to lead to a hyper contractile state of your heart.  

An analogy would be that you’re sitting still, but your heart is beating as if it's going for a jog. Over time, if you have that consistent stressor on your heart, it's going to enlarge, which isn't always a bad thing. But it can be when combined with fibrosis, the stiffening of the heart. Many times, these patients have a hard time pumping enough blood out of their heart to the rest of their body to get enough oxygen to their cells. Plus, they have a hard time relaxing their heart which is also a very important aspect.  


How common is hypertrophic cardiomyopathy in adults? 

Estimates are as high as one in 200.  Many are undiagnosed because they have no symptoms, or mild symptoms that might seem like asthma or being out of shape. It can be slowly progressive, so other people might just think they are getting older and might not realize that it’s not normal to feel lightheaded after an activity.  


As part of your BIO5 Postdoctoral Fellowship this year, you hope to travel to the University of Copenhagen and advance your laboratory skills. Can you tell us more about what you hope to experience? 

I'm super excited. We have a collaborator, Dr. Julian Ochala at the University of Copenhagen, who has developed a model where he can study the energy rates of the myosin heads.  

I've hinted at this, but in hypertrophic cardiomyopathy, those myosin heads are usually in this hyper contractile state, and they're likely using up a lot more energy than they need to. It's very energy inefficient. We suspect that if we cut out cardiac myosin binding protein-c in our different models, we would expect to see that it will be using up energy faster than ideal. 

I’m interested to learn more from Dr. Ochala’s lab because he is interested in a more disease-based approach. 


You mentioned using fluorescent microscopy techniques to show a relationship with muscle function. Can you talk a little bit about that? 

Yes, so Dr. Ochala’s lab uses fluorescent microscopy to estimate these different energy states of myosin.   

Essentially what we do isolate these little bundles of muscle and expose them to fluorescent ATP, which is the energy source. Then we wash it with regular ATP, which outcompetes for that fluorescently labeled ATP and you can watch the decay of the fluorescence. 

There are two general states, and this is a little bit controversial, but there's generally a state in which muscle uses up energy slowly, and then a state where it's using up its energy fast. We call that the super relaxed versus the disordered relaxed state, and I'm going to quantify those two states.   

I’ll collect the technique, collect some data, set it up in the Harris lab and hopefully my own lab someday! 


Can you give an example of something in your project or lab that really made you excited? 

I'm excited right now about my data set that I presented at the conference and received some excited responses.  

The middle portion of the protein seems to be important for moving one end of the protein away from actin. And without those domains or that segment of the protein present, it keeps activating actin, which means the probability of myosin and actin binding together and sliding producing force is higher. Which sounds like a great thing, right? You want more force, but in the heart, you also want to relax. So, it's a tradeoff.  


Do you have a mentor that has impacted your life? 

I would say my first mentor, Dr. Gary Diffee, from the University of Wisconsin was very influential on me. When I joined his lab, I thought I was just getting credits for medical school.  

He showed me that science can be fun, and you can also have fun outside of science. He was a great role model and taught me a lot about presentation skills. I hope to keep learning from him. 


What is your ‘why’? 

I noticed my ‘why’ is always changing. During my master’s, I started studying the effects of cancer on muscle function and muscle wasting, called cancer cachexia. And at the time, my dad had cancer. Studying that was so important to me, personally.  

My goal as a researcher is to better understand how diseases affect muscle function, in hopes that we could target it for therapy purposes. Now I’m focused on heart disease, and I’ve looked at the effects of diabetes on the heart. So, realizing that you can help individuals with your work is powerful.  

I notice as I get more senior, I believe I can make a big impact as a mentor myself. I've seen good mentoring and bad mentoring, and now I feel like I can be someone in the room who can help by being approachable, friendly, kind, and a good listener.  

I've hinted at how New Zealand was a great influence on me. We know that PhD students have a high rate of mental health issues, and I think sometimes the pressure they face from their mentors doesn't help. Those are my two ‘whys’ right now.  

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