Transcription
Imagine this tennis ball in my hand is a mitochondrion. The powerhouse of the cell. You'd think it just stays put, right? Born in one cell, dies in that same cell. But no, not even close.
Your cells are trading mitochondria, literally passing them around in a whole body mitochondrial economy that helps regulate your metabolism. It can boost your metabolic rate, and it might even protect you from a heart attack. When I read the papers I'm going to review with you today, I actually got goosebumps. For me, it's like imagining cars on a highway swapping engines while moving at 70 mph. Your cells and organs are constantly sharing and adapting in ways that reveal just how elegant, complex, and intelligent our biology really is. So, if you stick around, my goal is to make you feel the same awe I felt that I feel as we explore this brand new world inside you together.
Obesity disrupts mitochondrial exchange led to a decrease in energy expenditure. Different fatty acids have vastly different effects on mitochondrial transfer. Diet quality dictates how our bodies partition and use energy. It's not just about calories. I was not expecting that. What? Let's prepare the heart in case the worst happens. I'm getting goosebumps again. My jaw is on the floor.
So, let's begin with the first paper in our trilogy. This is the one that actually gave me goosebumps. It was published in Cell Metabolism. And now, paraphrasing from the authors, we the researchers found that macrophages, immune cells, acquire mitochondria from neighboring adipocytes, fat cells, in vivo in living organisms. And blocking this transfer of mitochondria to macrophages in fat tissue increases fat mass, lowers energy expenditure, and exacerbates obesity. I wanted to frame this with the author's own words, but don't worry if you didn't understand that. You're going to shortly understand that.
So the researchers, they already knew that different cells could exchange mitochondria. So then they set out to explore in this first of our three papers how this happens, how fat tissue in fat tissue, mitochondria are exchanged from the fat cells themselves to immune cells inside the fat tissue and how this impacts or is impacted by metabolic health and metabolic status. So to that end, they used emerging imaging technology to tag and track mitochondria in living animals. So what you're looking at here, this is amazing. It's a time-lapse image of a mitochondrion, which is singular mitochondria, the arrow, released from fat cells and being taken up by a macrophage, an immune cell. That's the red blob. We can visualize this mitochondrial transfer event in real time in living mammals. That's quite a feat of engineering.
Next, the researchers wanted to know how all this works. So, what they did is they used state-of-the-art gene editing technology, CRISPR-Cas9. Maybe you've heard of it. They then performed a knockout screen. So, they deleted a bunch of different genes to identify which were essential for the process of mitochondrial transfer between fat cells and immune cells to occur. And they ultimately landed on those genes related to the synthesis of something called heparan sulfate. This is a complex carbohydrate found on the surface of cells and in the extracellular matrix of all multicellular animals. It plays crucial roles in various biological processes including development, cell signaling, and immune responses. So effectively, heparan sulfate is the receptor, the bridge, and or the tool that the macrophages in fat tissue, the immune cells, need to take up mitochondria from fat cells.
Now, here is where things get extra interesting. Obesity disrupts mitochondrial exchange. So, when mice were made obese by a high-fat, high-sugar diet, levels of this heparan sulfate dropped, meaning the macrophages became handicapped at being able to uptake mitochondria. We'll discover where the mitochondria go later, but otherwise stated, in obesity, immune cells, macrophages in fat tissue are defective at taking up mitochondria. And as a consequence, energy homeostasis at fat tissue is disrupted. This leads to lower energy expenditure. Indeed, the researchers showed that blocking the mitochondria transfer from fat to immune cells in fat tissue led to a decrease in energy expenditure, decreased calorie burn in the whole animal, even independent of activity level. And this further exacerbates obesity. And then to complete the metabolic picture in this paper, they were able to show that inflammatory signaling molecules. So inflammation molecules like interferon gamma and lipopolysaccharide were sufficient to disrupt the mitochondrial transfer system as well.
So I know I said a lot. Let's step back. Here's how I interpret these results from the first paper. An obesogenic inflammatory diet like a Westernized standard American diet can alter immune cell function in fat tissue to disrupt mitochondrial transfer. This alters energy metabolism and can lead to fat gain, fat accumulation, obesity exacerbation, and also accumulated fat mass, specifically the large or fat cells, the enlarging of fat cells further exacerbates the problem, decreasing mitochondrial transfer and perpetuating fat accumulation. The simple truth I'm building towards is it's not just about calories. People always focus on calories, but it's always what the body does with the calories. Diet quality dictates how our bodies partition and use energy and even how they partition and use the energy engines of our cells at the fundamental level. How cool is that?
But we're only beginning now. We move on to chapter 2 in our mitochondrial transfer paper trilogy. In a subsequent study, also published in Cell Metabolism, researchers built on this prior work by investigating whether certain dietary components impaired mitochondrial transfer more than others. I'll trim the fat, so to speak, and get right to the point. The researchers found that in the context of a high-sugar, high-fat obesogenic diet, it was particularly the long-chain fatty acids that blocked mitochondrial transfer from fat cells to these macrophages. And interestingly, when mice were fed the high-sugar, high-fat obesogenic diets, if those diets were rich in longer-chain fatty acids, in this case taken from lard, so the lard diet, then heparan sulfate levels were reduced, mitochondrial transfer was impaired, and the mice gained more fat. And this was as compared to a similar diet but in which lard wasn't used but coconut oil was the fat source, which is lower in long-chain fats and higher in short-chain fats.
So just to be clear so you don't get confused, I'm not pointing the proverbial finger at fatty acids predominant in animal-based foods. The long-chain saturated and unsaturated fats, palmitic acid, stearic acid, oleic acid, and linoleic acid, all had similar effects in terms of blocking mitochondrial transfer from fat cells to macrophages. However, this is really striking. There were signs that the medium-chain fatty acids found in the coconut didn't decrease but increased mitochondrial transfer, which is the exact opposite of the effects of the longer-chain fatty acids. So the high-level point here is that different fatty acids have vastly different effects on mitochondrial transfer. These fats aren't just fuel. They're potent and unique signaling molecules that alter how our cells exchange mitochondria. Isn't that interesting?
But if the mitochondria aren't taken up by macrophages, where the heck do they go? The story continues to get more interesting. The researchers then found that when uptake of mitochondria by macrophages in fat tissue was impaired, it caused the mitochondria to spill over into the blood. What you're seeing here is mitochondria in the blood when mice were fed a high-sugar, high-fat lard diet versus a control diet or that high-sugar, high-fat coconut diet. Clearly, there's more mitochondrial spillage when the mice consumed the obesogenic diet, high-fat, high-sugar that was enriched in the longer-chain fatty acids, in this case from lard. And shown here, you see the mitochondria arriving in the hearts of these mice.
And that brings us to the third and final paper in our trilogy. What do mitochondria derived from fat cells do in the heart? In our third fascinating paper, also published in Cell Metabolism, yes, it's a Cell Metabolism hat-trick, researchers found that the high-fat, high-sugar diet caused obesity and it caused mitochondria to be transferred from fat tissue to the heart. So confirmation here where the mitochondria again from fat tissue that went to the heart, they triggered bursts of oxidative stress at the heart that rather than harming it per se, actually conditioned the heart to protect against heart attacks. What? I was not expecting that and I'm sure you weren't either.
I'm going to quote the authors because they put it brilliantly. They said, "This study provides the first description of functional mitochondrial transfer between tissues. The first vertebrate example of interorgan mito-phagy. Thus, these seemingly toxic adipocyte small extracellular vesicles containing mitochondria may provide a physiologic avenue of potent cardioprotection against the inevitable lipotoxic or ischemic stresses elicited by obesity." Now, there are a lot of $10 words there, but to put it simply, the researchers are hypothesizing that transfer of functional but damaged mitochondria from overfilled enlarged fat cells to the heart conditions heart tissue in preparation for the inevitable and potentially fatal event to come. It's as if the body were saying, "Okay, we're obese. That's not good." and the excess fat might cause a heart attack. So, let's prepare the heart in case the worst happens.
In effect, the researchers were able to show that the oxidative burst caused by mitochondria coming from fat tissue that ended up in the heart regulates the heart's antioxidant defenses. Then when a heart attack does occur, in this case they induced it in animals, the heart is better prepared and ends up with less dead tissue, less infarcted tissue. It's absolutely remarkable. I'm getting goosebumps again.
And as to whether this is relevant in humans, the researchers also found that in metabolically unhealthy obese individuals, levels of the vesicles containing these mitochondria that were spit off from fat cells were indeed enriched in the blood of these metabolically unhealthy individuals as compared to lean healthy individuals and as compared to metabolically healthy obese individuals.
Just pause the video now, but just to like remark, like reflect on how amazing this is. But a question that lingers for me, and maybe you too, is how did evolution select for this response? If, in fact, this is an adaptation, I guess we can't rule out the possibility that it's just a remarkable accident of biology, but it's so phenomenally elegant that my jaw is on the floor. I don't fully understand it, but I am awestruck.
Now, taking a breath here, I'm getting myself worked up. As groundbreaking as these research findings are, we have to recognize their limitations. Much of this work was, and necessarily was, done in mice. The mechanisms observed, they're elegant and compelling, but they still need to be validated in humans in different contexts and over time. We don't yet know how these mitochondrial exchanges scale across tissues beyond fat tissue, maybe in brain tissue, in real-life human physiology, or how they interact with the layer complexities of different elements of diet, genetics, environment, and disease. So, no, we can't yet turn this science into prescriptive action. And there's no clinical test to optimize your mitochondrial transfer rate. There's no supplement or intervention proven to tweak the system in an adaptive manner.
But that doesn't mean these insights are any less valuable. Because what this body of research ultimately gives us is awe and a deepened respect for the incredible adaptive mechanisms, maybe adaptive mechanisms, that occur in our bodies. Remember, the goal of this video was never to hand you a formula for "do X to get Y." It was to invite you to a goosebumps moment. The sparks of wonder I feel reading these studies. Tell me, do you feel it? I hope you do. If so, I did my job. Stay curious and thank you. [Music]