From the first flutter of cells in an early embryo to the pounding pulses in a marathon runner, heart muscle cells must beat for a lifetime. If that rhythm sputters off beat even briefly, the consequences can be deadly.
Heart disease is the leading cause of death worldwide, yet scientists still know little about the cells that maintain that life-sustaining beat: cardiac myocytes. During development, heart muscle cells divide to make more of themselves, but soon after birth, they mostly stop dividing and simply grow in size. Exactly how this process occurs and the identities of the signals involved remain a mystery.
“During development, normally, how does a heart muscle cell get bigger?” asked Dylan Burnette, a cell biologist at Vanderbilt University. “I was a bit shocked when I learned that we don't know the answer to that question on a basic level.”
He has since made it his mission to find out. Using cell culture systems, animal models, and microscopy, Burnette studies the molecular mechanisms that underly heart muscle cell development and growth. His microscopic images of heart muscle cells have not only won awards such as Nikon’s Small World Photomicrography Competition, but have also inspired Burnette to pursue new translational research questions and to investigate treatments for cardiovascular disease.
What sparked your interest in heart muscle cells?
I went to graduate school, and I fell in love with microscopes. It's fairly easy to become a cell biologist if you love microscopes. At that time, I had just gotten engaged to my wife, but we had not told anyone yet because our best friend's wedding was the next week. She went to the wedding and hung out with her brother, which was awesome because she didn't get to see him often. The next week, he died of a heart attack.
He had an undiagnosed hypertrophic cardiomyopathy, which is where the heart muscle wall, usually of the left ventricle, gets so thick that it causes electrical or valve problems in the heart. It is fairly common in older people, but when young people have it, which is rare, they typically die. It's pretty traumatic because it's very difficult to diagnose. There are no symptoms.
As a cell biologist, I had never even heard of hypertrophic cardiomyopathy, but on a cellular basis, the condition arises when heart muscle cells get too big. That's what generated my interest in muscle cells. I would never have worked on this question of how heart cells get bigger if my family had not gone through that event.
What do we know so far about how heart cells grow?
Heart muscle cells only have two basic things inside the cytoplasm: mitochondria and sarcomeres. The mitochondria generate ATP for the cell, and the sarcomere, which is a repeating unit that generates the contractile force for your muscle, burns through ATP like it's going out of style. When the muscle cell gets bigger, the cytoplasm gets bigger, and the cell makes more sarcomeres and mitochondria.
In my lab, we study how sarcomeres form, the biogenesis of mitochondria, and how those two things are related because they communicate with each other quite a bit. When we go out for a run, our heart muscle cells get bigger and stronger. That’s called a healthy hypertrophy, and that’s good. But then if we go sit on the couch to watch TV, our heart cells get smaller. It can go both ways, and that relationship must be well coordinated.
How do you study how heart cells grow?
We use both cell culture systems and zebrafish models to study how sarcomeres and mitochondria form in muscle cells. The nice thing about zebrafish is that we can image their hearts in live animals and study how cells develop in different chambers of the heart. Some of our in vitro work predicted the heart getting bigger, but we saw that in vivo, only the atria got bigger. We are now diving down into how different chambers of the heart grow over time.
It is an academically satisfying thing to do, but is it going to cure disease? I don't know. For many years, I thought about whether I was satisfied with that. It turned out that no, I'm not satisfied with that. We are now developing more medically relevant translational assays.
What kinds of translational assays are you working on?
We decided to look into what kills most people: acute myocardial infarctions. During a heart attack, there is a reduction in blood flow from an artery that has become clogged. The loss of blood flow starves a part of the heart muscle of oxygen and nutrients, and that heart muscle dies as the surrounding tissue begins to die. If people live close to a big hospital, doctors can mechanically remove the clot, put a stent in the artery, and do something called reperfusion to restore blood flow to the heart tissue.
Cardiologists have a cute little saying about that process: Time is muscle. The faster doctors can remove the clot and restore blood flow, the more muscle they can save. Unfortunately, people not living close to hospitals don’t have that luxury. There are drugs that will dissolve a clot over time, but because a clot can take a long time to dissolve, the heart muscle continues to die.
I learned early on that heart muscle doesn't just die when there is a reduction in blood flow. Additional heart muscle dies during reperfusion. The muscle gets used to having no oxygen around, and when the cell uses oxygen to make ATP, the oxygen actually damages the cell.
We designed both in vitro and in vivo assays to screen small molecules that keep our muscle cells alive when there's no oxygen or nutrients around. We have about 20 hits from our initial screen, and now we're testing our candidates in a mouse model of ischemia reperfusion injury — basically mouse heart attacks — in collaboration with Jitka Virag at Eastern Carolina University.
We have to figure out how to administer the drug via the bloodstream and get it to heart muscle that has very little blood flowing to it. We hope to find a potent drug or drug combination that is nontoxic at high levels in the bloodstream and only effective at low levels. Hopefully this drug will get to the affected muscle tissue and keep it alive while a person receives a stent or clot dissolving drugs. It would be great if people had access to cheap, effective drugs while they recover from a heart attack.
You take beautiful photos of cardiac myocytes and other cells. How did you become interested in microscope photography?
That was almost random. Most data we collect are just data, and the beauty from that data is represented mostly in graphs, not in the actual images. But one day, I was like, "Oh, aren't these pretty?" And that was it. After a while, I gained a skill set to make images with pretty colors. When the data are aesthetically pleasing, I make a copy of the raw data and move it into a folder on my computer just for art. The images that come out are pure artistic expressions of the data.
Have any of your artistic microscope photos inspired new research questions?
Yes! For the purposes of art, I've only been focusing on the bright, pretty things, but there are a lot of interesting dim objects. I was doing electron microscopy, and I saw a bunch of actin filaments that I wasn't thinking about at all. I went back to the images I used for art and blew them up. There's an entire actin cortex there.
The most interesting aspect of the cortex for us is during cell division. I asked a colleague who is an expert in cytokinesis about how arrays of myosin two formed in the cleavage furrow, the part of the cell that pinches off the two daughter cells during cell division. And he asked, “What arrays of myosin are you talking about?” We discovered a phenomenon that people hadn't published before because we looked at dim stuff.
What do you find most exciting about studying heart cell growth?
There are few cell types that are aesthetically pleasing and important to study. Heart muscle cells are both. Many cell types are important to study, such as cancer cells, but most of them are actually rather ugly. They look like they would kill someone. For heart muscle cells, our end result is a symmetrical, almost crystalline organization of the cytoplasm. Our goal is to understand something that's inherently beautiful.
This interview has been condensed and edited for clarity.