Microscopic 'traffic jams' studied for possible cardiac effects

Host Amber Smith: Upstate Medical University in Syracuse, New York, invites you to be The Informed Patient, with the podcast that features experts from Central New York's only academic medical center. I'm your host, Amber Smith.

The campus of an academic medical center, such as Upstate Medical University, contains a multitude of researchers, and today we'll meet one of them who is studying the quality control of mitochondria for a healthier heart.

Ms. Gargi Mishra is a graduate student in biochemistry and molecular biology at Upstate in the laboratory of Dr. Xin Jie Chen, working toward her MD and PhD degrees.

Welcome to "The Informed Patient," Ms. Mishra.

Gargi Mishra: Thank you so much for having me.

Host Amber Smith: Let's start with some basic definitions. What are mitochondria?

Gargi Mishra: Mitochondria are organelles within our cells.

So if we can think of our cell as having a plasma membrane and various different things within it that help that cell make energy, help translate DNA, help divide, mitochondria perform the job of helping produce energy.

And if we were to go a step back, mitochondria weren't always in our cells, and our cells weren't always modern, the way they are (now).

Many, many, many, many, many years ago, a precursor to our cell, a eukaryotic cell, engulfed a very ancient bacterium. And rather than just eating at that bacterium and digesting it, that bacterium took a place up in the cell and became what is called an endosymbiont and began to provide energy for the cell.

And that, in modern times, became the mitochondrion within our cell, or, like, multiple mitochondria within our cell.

Host Amber Smith: So our cells have evolved over centuries to have mitochondria. And this is in all the cells of our body, right?

Gargi Mishra: Yes. This is in all the cells of our body because we're a eukaryotic organism, all our cells, and eukaryotic cell essentially means that it has its DNA packaged into a nucleus, which is again, membrane bound, whereas bacteria are prokaryotes, so sure, they have DNA, but it's just not membrane bound into a nucleus.

Host Amber Smith: Now, are mitochondria important because of their energy function? Is that the big thing about them?

Gargi Mishra: That is what they are known for the most. If you think about textbooks and what students learn in middle school or high school, you know the phrase is: Mitochondria are the powerhouses of the cell. So there's certainly those energy hubs that help generate energy for the cell, but they are extremely important for other functions that are just lesser known.

So they're important for generating various metabolites that the cell uses. They're essential for immune signaling. They are important for signaling associated with cell death and cell proliferation, the balance of calcium within our cells, and that's especially relevant for different types of cells, such as heart muscle or skeleton muscle.

The way those types of cells contract and relax, to a huge degree, depends on the calcium levels within them. So, yes, mitochondria perform many other functions besides producing energy, but that is what they're known for the most.

Host Amber Smith: You used the term "metabolite." Can you explain what that is?

Gargi Mishra: Metabolites are basically molecules that enzymes will act on to either synthesize into, like, a larger metabolite or break it down into energy. So an example of a metabolite could be glucose, or it could be acetyl-CoA, or various other molecules that can be used for energy purposes.

Or it can be shuttled off to outperform another function in a different pathway.

Host Amber Smith: So what are the functions that relate to the heart?

Gargi Mishra: I think the most salient functions that mitochondria perform in the context of the heart, first and foremost, for sure, is energy production. The heart is an organ that is working 24/7, nonstop, even before our parent gave birth to us.

And until we take our last breath, our hearts are pumping blood, and so they require huge amounts of energy, and our heart especially utilizes fatty acids as the preferred substrate (source) for energy production and that generates ATP (adenosine triphosphate), which is the energy currency of the cell.

And so mitochondria are relevant for that, first and foremost. And then again, as I mentioned, they're important for metabolite synthesis. So various types of cholesterols, such as cardiolipin, acetate and various other metabolites, are generated with the help of pathways that occur within and surrounding mitochondria.

Mitochondria are important for calcium homeostasis, which, again, is relevant for heart function and the way the heart cell is able to contract so that the organ is able to pump blood.

And then, if you were to think about slightly lesser-known functions, mitochondria are important for what is called this "redox balance." Taking a step back, while mitochondria are helping produce energy, that process generates what are called these free radicals or, reactive oxygen species. And, because these can be generated by huge amounts. these can lead to a lot of what is called oxidative stress within the cell. And so, you would think that mitochondria are the ones producing it, that they're causing the trouble. But they also have the enzymes that can help to disperse and reduce some of these reactive oxygen species, or ROS, as they're called.

And then finally the even more lesser two known functions. One would be what is called mitochondrial dynamics. So mitochondria can undergo fission and fusion. Fusion refers to multiple different tiny mitochondria fusing into one large mitochondrion, or they can undergo fission, where that larger mitochondrion can break up into tinier mitochondria, and that can help redistribute energy and metabolites within that heart cell.

And then finally, the least studied or understood function in the context of the heart is just the production of mitochondria in timely fashion as the heart cells are doing what they're supposed to do, and also the clearance of damaged mitochondria or of proteins that may accumulate outside of those mitochondria. And that we refer to as mitochondria biogenesis, which is just the production of mitochondria and mitochondrial quality control, which would be the turnover of mitochondria. And those things are also relevant for heart function.

Host Amber Smith: You used the term "redox." Can you explain what that is?

Gargi Mishra: It's a combined term for reduced versus oxidative, and that redox balance, in the context of mitochondria and what they do, is essentially referring to the amount of reactive oxygen species that are being produced as mitochondria help generate energy.

If you were to get into the nitty-gritty of it, you can have a more reduced state in the cell. It's basically referring to charges in or around the mitochondria. A reduced state would be having more protons, like minus charges, and the oxidative state would be having fewer protons.

Host Amber Smith: This is Upstate's "The Informed Patient" podcast. I'm your host, Amber Smith.

I'm talking with Gargi Mishra. She's in the MD/PhD program at Upstate, working in biochemistry and molecular biology.

So if I understand correctly, mitochondria have already been targeted as treatment areas for diseases of the heart muscles, such as cardiomyopathy; arrhythmias, where the heart's electrical system is malfunctioning; heart attacks; heart failure.

So that's kind of already been studied quite a bit, right?

Gargi Mishra: Yes, for sure. Mitochondria have been targeted in these things, and it makes sense why. Various genetic mutations in mitochondria can directly lead to cardiomyopathies or various forms of arrhythmias and also other types of insults, whether it be lifestyle induced based on someone's diet or the toxins they're exposed to, or various different drug treatments can induce cardiomyopathies that also indirectly cause these various diseases, via causing mitochondrial dysfunction.

But the way in which mitochondria have been targeted has overemphasized their bioenergetic function. So supplementation of various metabolites, such as, let's say, acetyl-CoA or NAD (nicotinamide adenine dinucleotide) or various antioxidants that have been used, are all kind of overemphasizing the energy production function of mitochondria. Or calcium homeostasis function of mitochondria or the redox balance or, to a lesser degree, targeting mitochondrial dynamics function of those organelles.

But what remains understudied or underutilized as an approach for targeting mitochondria is simply mitochondrial production and mitochondrial quality control.

Host Amber Smith: That's what you're looking at, right?

Gargi Mishra: Yes. That is something that our lab collectively has been interested in for a very long time, and I would like to quickly give a shoutout to an older graduate student, Liam Coyne, MD, PhD, who's currently in his intern year at Johns Hopkins, pursuing a physician-scientist training program there. And Liam's PhD focused on, first and foremost, establishing how defects in mitochondrial production, simply from mitochondrial protein import not occurring, can lead to this phenomenon called protein import clogging on the mitochondrial surface, and how that can be used as a way to explain the mechanism for various diseases.

And then if we were to go even a step back, my PI (principal investigator), Dr. Xin Jie Chen, his work, I would say in around 2015, helped establish this form of protein-induced stress, which he termed "mitochondrial precursor over accumulation stress," or mPOS, which, again, is caused when proteins that are supposed to end up in mitochondria because they help form mitochondria are not able to get in. They cause this traffic jam on the surface of those mitochondrial import channels, and then the accumulation of these proteins in the cytoplasm can lead to stress within the cell and eventually lead to cell death.

And so, based off of what Dr. Chen found and what Liam found as, like, a new form of mitochondrial dysfunction, my project is focused on finding ways in which we can reverse mPOS, or ways in which we can reverse clogging, which we refer to as trying to identify novel anti-clogging pathways.

Host Amber Smith: So you've identified some problems, and now you're trying to figure out a way to solve those issues.

Gargi Mishra: Yes, that's exactly right.

Host Amber Smith: So protein import defects and protein import stress, do those things affect only the heart, or are there other areas of the body where that's a problem?

Gargi Mishra: That's a very good question. My project and another lab mate's project is focused on looking at mitochondrial protein import stress and how it can relate to the heart.

But we have other models in our lab, like we have a lab mate who's looking at mitochondrial protein import stress with regards to skeletal muscle dysfunction and another colleague who is focused on, and will be focused on, looking at protein import stress with regards to neurodegenerative diseases.

You could truly argue, like, any cell in the body, if there's a defect in how mitochondria are produced, and they require these thousand different proteins to come into them to actually form what is a mitochondrion. If that doesn't occur, of course mitochondria can perform their function, but also these proteins accumulate in that cell.

And then, for the cell, if they are just acting as, like, trash, because they don't serve a purpose in the cytoplasm -- they would've served a purpose if they got into mitochondria -- and so the cell has to then hurry to try to degrade these proteins. But at some point, if the traffic jam is too intense, and these proteins continue to accumulate and become toxic aggregates, it can affect the viability of that cell.

And so technically, like, this kind of protein import dysfunction can affect any cell type in the body. But specifically, whether it be mouse models or in yeast, which are unicellular organisms, we only study in those contexts.

And another point to make is that the reasons where our lab particularly is focused on, let's say, the heart cell or a skeletal muscle cell or neuronal cells, because these are three types of organ systems that depend on large amounts of energy to function and also on large amounts of metabolites and turnover.

Host Amber Smith: Commonly, we think of stress as being a negative. Is mitochondrial protein import stress always bad for the heart?

Gargi Mishra: No. That, again, is a very excellent question. The type of mutant protein that I use as a model for studying protein import clogging in yeast, it translates to a mutant protein that does in fact cause cardiomyopathy in humans.

And again, our hypothesis for how this happens is because this mutant protein is getting stuck on protein import channels and preventing the import of other mitochondrial proteins from getting in.

That being said, we have a mouse model in our lab where we're just over-expressing the healthy version of this protein (not mutated, as it would be in a disease) by about twofold, and in that mouse model, which is something that my colleague Arnav Rana is working on -- he's also an AHA (American Heart Association) funded research fellow in our lab -- what he's actually found is that over-expressing two copies of just the healthy protein that we want to import through the channel, causes extremely mild levels of protein import stress, and this translates to improved heart function.

And this is an extremely paradoxical finding, but it's also not too unsurprising because if we were to think of it from the perspective of doing exercise, so let's say if you're doing cardio, you're going on a long run, you're biking every day, or you're swimming, you're basically pushing your heart to try to keep up with the, let's say, the energy demand or the metabolite demand for it to perform its function. And in those cases, mitochondria production or mitochondrial biogenesis is actually upregulated to some degree. And so you can imagine that if you're trying to produce more mitochondria, you're trying to do more protein import.

And so while all these proteins are trying to be imported in, so new mitochondria can form, at some point, there can be just a mild level of traffic jam. Think about it as, like, rush hour, on I-81, right? It's not necessarily a complete traffic jam because a car crash happened, but it's just a lot of cars trying to get to where they need to go. And so that process of doing exercise translates to what we think, we speculate, it translates to low levels of protein import stress that ultimately, through various signaling pathways, leads to a stronger heart, improved ejection fraction (how much blood the heart pumps), which is the readout that Arnav uses. And so Arnav, in his PhD, is trying to identify what are those signaling pathways downstream of that mild protein import stress that are leading to this healthier phenotype (observable characteristics) in these mice.

Host Amber Smith: Well, very interesting. I want to thank you for making time to tell us about your research.

Gargi Mishra: Thank you so much, Amber.

Host Amber Smith: My guest has been Gargi Mishra, a fellow in biochemistry and molecular biology at Upstate, working on both her MD and PhD degrees.

"The Informed Patient" is a podcast covering health, science and medicine, brought to you by Upstate Medical University in Syracuse, New York, and produced by Jim Howe.

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