Jul 16, 2020
This month on Episode 14 of the Discover CircRes podcast, host Cindy St. Hilaire highlights four featured articles from the July 3 and July 17 issues of Circulation Research. This episode also features an in-depth conversation with Dr. Brenda Ogle and Drs. Molly Kupfer and Wei-Han Lin regarding their study, In Situ Expansion, Differentiation and Electromechanical Coupling of Human Cardiac Muscle in a 3D Bioprinted, Chambered Organoid.
Article highlights:
Wei, et al. Palmitoylation Cycling and Endothelial Maturity
van Ouwerkerk, et al. Functional Variant Elements in Atrial Fibrillation Models
Ibarrola, et al. Aldosterone in MVP
Sharma, et al. Atherosclerosis
Regression Requires Regulatory T Cells
Cynthia St. Hilaire: Hi, welcome to Discover CircRes, the podcast of the American Heart Association's Journal, Circulation Research. I'm your host, Dr. Cindy St. Hilaire, from the Vascular Medicine Institute at the University of Pittsburgh. Today I'm going to share with you four articles selected from our July issues of Circulation Research, as well as have a discussion with Dr. Brenda Ogle and the first authors, Molly Kupfer and Wei-Han Lin, regarding their study, In Situ Expansion, Differentiation and Electromechanical Coupling of Human Cardiac Muscle in a 3D Bioprinted, Chambered Organoid. So first, the highlights.
The first article I want to share with you is titled, "Endothelial Palmitoylation Cycling Coordinates Vessel Remodeling in Peripheral Artery Disease." The first author is Xiaochao Wei, and the corresponding author is Clay Semenkovich from Washington University, St. Louis. Peripheral artery disease, or PAD for short, is a vascular occlusive disease of the lower extremities. It affects more than 2 million individuals globally, and its prevalence is ever increasing as our population ages. While statin therapy can be useful for combating coronary artery disease in peripheral artery disease patients, it does not prevent or reduce PAD patients' rates of lower extremity amputation.
So looking to gain insights into the mechanisms underlying PAD, this team focused on the findings that circulating fibronectin and the dietary saturated fatty acid, palmitate, are associated with peripheral artery disease. They found this interesting as lipid modification proteins has been implicated in infections, premature aging, cancer and diabetes. One such protein modification is palmitoylation, which is the formation of a thioester bond between palmitate sand cysteine. Acyl-protein thioesterase 1, or APT1, is a depalmitoylase enzyme, which removes the fatty acid palmitate from protein.
Using mouse models with inactivated endothelial APT1, as well as cell systems in arterial samples from humans with end stage peripheral artery disease, they tested whether deficiencies in palmitoylation cycling promotes endothelial instability, which is a hallmark of chronic arterial occlusive diseases. They discovered that as many as 10% of all proteins are palmitoylated. They found deficiency of APT1 in endothelial cells disrupts vascular homeostasis, in part by altering the intracellular trafficking of the small GTPase R-Ras. Impaired R-Ras membrane trafficking was rescued by modifying the palmitoylated R-Ras molecule to promote dissociation from membranes. These observations identify palmitoylation cycling as a potential therapeutic target in the treatment of peripheral vascular disease.
The second article I want to highlight is titled, "Identification of Functional Variant Enhancers Associated with Atrial Fibrillation." The first author is Antoinette van Ouwerkerk, and the corresponding authors are Antoine de Vries and Vincent Christoffels, And they're from UMC Amsterdam. As we heard in our podcast last month with our interview with Dr. David McManus, atrial fibrillation, or AFib, is the most common form of arrhythmia, and is a major risk for heart failure, dementia, and stroke, and sudden death. Genome-wide association studies have revealed more than a hundred genetic loci linked to this condition, and many of these loci are found in non-coding regions, which are enriched for transcription factor binding sites and epigenetic modification sites, suggesting that these loci could potentially have gene regulatory roles.
To test this idea, they use the method called self-transcribing active regulatory region sequencing, or STARR-seq, which is a method used to identify the sequences that act as transcriptional enhancers in a direct quantitative and genome-wide manner. They use STARR-seq to screen 12 of the strongest AFib linked regions of the genome, which contain more than 1600 individual aphid linked genetic variance, and they did this in cultured rat atrial monocytes. From this screen, they found approximately 400 regulatory elements, of which 24 exhibited variant-specific differences in regulatory activity. For one of these elements, upstream of the gene HCN4, deletion of the orthologous element in mice caused diminished transcriptional activity of the gene. Moreover, these variant-containing mice had brachycardia and sinus node dysfunction, both components of arrhythmia. This proof of principle study confirms that such a regulatory element screen could provide insight into the consequences of variants associated with AFib, or for that matter, many other diseases.
The next article I want to share with you is titled, "A New Role for the Aldosterone/Mineralocorticoid Receptor Pathway in the Development of Mitral Valve Prolapse." The first author is Jaime Ibarrola, and the corresponding author is Natalia López-Andrés, and their work was completed at Sanitaria de Navarra in Pamplona, Spain. Mitral valve prolapse is a condition where blood leaks back into the left atrium of the heart, and it is the most common form of heart valve defects. The underlying pathology includes an overabundance of cells in the valve leaflet, so-called valve interstitial cells, or VICs. These activated VICs overproduce extracellular matrix protein, and the combination of increased numbers of VICs and increased amounts of extracellular matrix proteins contributes to the impairment of the structural integrity of the valve leaflet. The increase in VICs is due to excess proliferation, but also transformation of valve endothelial cells, so the cells that line the leaflet, valve endothelial cells, into mesenchymal like VICs.
As a driver of endothelial to mesenchymal transition, aldosterone was suspected to play a role. Aldoesterone increased expression of VIC activation markers in cultured valve endothelial cells and increased production of certain extracellular matrix protein components. Spironolactone, an aldosterone inhibitor, prevented these effects, and importantly, prevented valve remodeling in a mouse model of mitral valve prolapse. The team showed that valve tissue from mitral valve prolapse patients taking aldosterone receptor inhibitors displayed less evidence of VIC activation and lower production of disease-regulated extracellular matrix components, than those not taking the drugs. These exciting results suggest aldosterone antagonists, already used for certain patients with heart failure or high blood pressure, may also benefit those with mitral valve prolapse.
The last article I want to share before we switch to our interview, is titled, " Regulatory T Cells License Macrophage Pro-Resolving Functions During Atherosclerosis Regression." The first author is Monika Sharma, and the corresponding author is Kathryn Moore, and they're from New York University. Atherosclerosis is a chronic inflammatory condition characterized by the buildup of fatty deposits in the artery walls, and monocytes and macrophages can infiltrate into these fatty deposits and contribute to the formation of plaque. Cholesterol-lowering drugs, like statins, promote the reduction of low-density lipoproteins in the blood, which can help to slow plaque growth, but they do not reverse disease progression.
One possibility for changing the course of the disease is to develop therapies that can reduce plaque inflammation, and therefore, progression. With that goal in mind, this team investigated how the immunosuppressive activity of regulatory T cells, or Tregs, may influence the functions of plaque monocytes and macrophages. Using mouse models in which the disease can be reversed through aggressive lipid lowering, they found that depletion of the Treg population caused an increase in the numbers of monocytes and macrophages in the plaques, and resulted in poorer plaque regression. Indeed, these monocytes and macrophages proliferated more, remained in the plaques longer, and were less likely to adopt an anti-inflammatory pro-plaque resolving M2-like phenotype than plaque macrophages in mice with normal Treg numbers. Together, these results highlight the importance of Tregs for promoting plaque regression, and suggest future therapies aimed at boosting these cells, or indeed, M2 macrophages may enable atherosclerosis remission.
Okay, so now we're going to switch over to the interview portion of our podcast. I have with me Dr. Brenda Ogle, who is a professor of biomedical engineering, and first authors Molly Kupfer and Wei-Han Lin, and they're from the University of Minnesota. And today we're going to be discussing their manuscript titled, "In Situ Expansion, Differentiation and Electromechanical Coupling of Human Cardiac Muscle in a 3D Bioprinted, Chambered Organoid." So thank you all for joining me today.
Brenda Ogle: Thank you.
Molly Kupfer: Thanks for having us.
Wei-Han Lin: Thank you.
Cynthia St. Hilaire: Great. I'm glad we can all do this remotely and nice and safe for COVID.
So Dr. Ogle, you're the PI of the group, but Molly and Wei-Han, what stages of career are you at?
Molly Kupfer: I just recently completed my PhD, so this work is sort of the culmination of that.
Cynthia St. Hilaire: Oh, congratulations!
Molly Kupfer: Yeah. Thank you.
Cynthia St. Hilaire: Well done. Circ Research is a great thesis publication. Congratulations.
Molly Kupfer: Thank you.
Cynthia St. Hilaire: Wei-Han, how about you?
Wei-Han Lin: So I'm a BME PhD student at the University of Minnesota. And I got my master degree in chemical engineering, but in Taiwan, and now I'm working with professor Brenda Ogle on cardiac tissue engineering stuff.
Cynthia St. Hilaire: Excellent. So this is a beautiful paper. It's stunning. It has all sorts of wonderful parts, biological, biomechanical, great imaging, and essentially you created a 3D bio-ink that can be used to print and make a living pump, kind of a heart in a dish. And it's something that you're calling this human chambered muscle pump, or ChaMP, which I think is a great name. Can you please describe exactly what that is and why did you want to go about trying to make it?
Molly Kupfer: Yeah, it might help if I give a little bit of context to this. So since the beginning, one of the central questions that the lab has been exploring is how do the cells of the heart interact with their environment, or the extracellular matrix, as we call it? We know that these interactions that occur at the cellular level are absolutely critical for cardiac function, both at the tissue and the organ level. And based on years of research studying how the extracellular environment modulates cellular function, we have now sought to apply what we've learned in order to engineer functional human cardiac tissues by recapitulating those very critical interactions in vitro.
And actually, back in 2017, we published another study in Circulation Research, where we generated these contractile patches of cardiac tissue using a form of light-based 3D printing that allowed us to fabricate scaffolds with really high resolution micron-level features that were distributed in a way that mimics the native extracellular environment. And what we found is that by organizing the extracellular matrix in that way, we enabled the cells to organize themselves in the scaffold and form connections with each other and with the scaffold itself. And this was critical to achieving synchronous electromechanical function of the tissue as a whole. But these were very small millimeter scale tissues, and so for this new study, we sought to create something on a larger scale where you could incorporate some new geometric features such as chambers and the capacity for perfusion.
And as you mentioned, using our knowledge of the interactions between cells and the extracellular matrix, we developed this unique bio-ink that could be used as a vehicle to 3D print these centimeter scale chambered tissue structures that are based on the geometry of the human heart. And so the tissues that resulted from this, the human chambered muscle pumps, or hChaMPs, exhibit thick, contiguous muscularization. They demonstrate electrical connectivity and pump function. And notably, this is the first time that this level of function and muscularization has been achieved in an engineered cardiac tissue of this level of geometric complexity.
Cynthia St. Hilaire: So can you maybe talk a little bit about what do you mean by an ink, exactly? Is it actually printed? Is this like a printer that I could buy on Amazon? Obviously there's a huge biological component, but what are the actual technical things that you had to develop to make this chamber happen?
Molly Kupfer: Yes. So we did use an extrusion-based 3D printing, which is similar to probably what people normally think about with 3D printing. Traditionally, it's been with plastics. In this case, we're printing with a bio-ink, which is essentially a formulation of proteins and other materials that we encapsulate the cells in, and then after that, we extrude it from a nozzle in a specific formulation or shape in order to create the structure.
Cynthia St. Hilaire: So that's interesting. So in this mix, the cells are already in there as opposed to, I guess, some other things that people tend to call scaffolds where you kind of print that and then seed it?
Molly Kupfer: Mm-hmm (affirmative). And in the example of the paper I discussed from 2017, that was an example where we printed a scaffold and put the cells in. But in this case, for such a large and complex structure, we actually mix the cells in prior to printing, and then we create the structure.
Cynthia St. Hilaire: Wow. What's the timeframe of that? Like the cells, you got to digest them and mix things up and then print it. The cells, are they happy?
Molly Kupfer: Yeah, that's a good question. So the actual printing process is quite fast, maybe a couple of minutes for this particular scale. We have to prepare, culture, the cells in advance and we're working with human-induced, pluripotent stem cells, so it takes time to grow them up, and then yes, we do detach them and singularize them, and we then mix them with the components. But overall, the actual printing process is relatively quick. Then it's a matter of maintaining the structure and culturing it and doing the differentiation as we did. And that takes weeks to do over time. But the actual process of making it, initially, is quite quick.
Brenda Ogle: Challenging thing about this project was the fact that mature cardiac muscle does not transfer well. Meaning when you move it from a dish to an ink and then print it and ask it to start beating again, it doesn't typically happen. And that is because cardiomyocytes don't proliferate well, or make more of each other, and they also don't move well, or migrate. And so the premise on which most of this paper relies is on printing the stem cells first, letting them expand, sort of like they do with development, and then encouraging them to specify into cardiac cell types.
Cynthia St. Hilaire: What's the bigger good that can come out of this? Why do we want to be able to do this in vitro, or even ex vivo heart in a dish?
Brenda Ogle: The value is pretty tremendous because, suddenly we have a human model system in which we can perfuse volume, so volume can go in and come out, in which the cells experience those volume metric and fluid-induced forces that we haven't been able to study human cells in this way ever before. In the context of human disease, this is the first time we'll be able to look at onset of a particular disease, what was happening with onset, and then progression. And I think that is what is going to transform this field.
Cynthia St. Hilaire: So what was the first one like? I'm thinking back to my graduate school and also my postdoc where I was involved in some disease discovery and I have a very vivid memory of the Western blot that proved the mutation that we found. And I literally ran down the hall holding the film. I'm imagining, maybe I'm projecting too much, but what was seeing that first one beat like?
Molly Kupfer: You're not projecting. I feel like that well describes my experience. We had some early experiences where we would start to see beating areas under the microscope, but I think the moment, for me, was, I think there was one night I was working in the lab and I had some plates out, I was looking at stuff under the microscope going through just the mundane lab tasks, and I think I sort of saw it at the corner of my eye in the dish, something was moving. And that was the first time. Like I had watched parts of these things beat under a microscope all the time. I spent years looking at cardiomyocytes under a microscope, but that was the first time, for these hChaMPs, where I could actually see it moving just by my eye.
Cynthia St. Hilaire: Wow.
Molly Kupfer: And that was a really cool moment.
Wei-Han Lin: Yeah. I was mostly working on the printing side, so the first time I realized the heart started beating, it's more like a shock to me, because I'm always printing the models or just the mold. But then really seeing those cells, or the whole structure, start to beat, was quite amazing.
Cynthia St. Hilaire: Could you please tell me a bit about the 3D printing aspect of it? Is it like a shell like the outside of a balloon, or does it have an interior structure that helps dictates where the cell go? Can you explain what the printing is?
Wei-Han Lin: So the structure we are printing is derived from MRI image stacks on a real human heart. And the image stack was segmented and reduce the size by 10 times, and then we convert the stack into the STL file, which is the standard operating format. And then we modify the model a little bit to make it into two chambers and with two vessels, and two connected chambers with two openings. And this is the heart we are using for the study.
Cynthia St. Hilaire: Got it. So it's got kind of the big picture items of the heart. It's got two tubes going in and it's got two chambers and the fluid can flow between all of those aspects in a specific flow pattern.
Wei-Han Lin: Exactly.
Cynthia St. Hilaire: You said you have to differentiate them in a dish and you're adding different factors to do that. Do the cells like being in that scaffold, or do they want to seep out of that structure or is there something about the bio-ink that they're happy there?
Molly Kupfer: You know, I think this bio-ink was, to a certain extent, optimized or designed such that the cells would be able to continue to attach and grow and remodel. So basically, for the most part, these components are biological materials. Some of them are just proteins. Some of them are proteins that have been modified with photo cross-linkable elements, but they still have these moieties that the cells can attach to. And over time we do see some remodeling and some extracellular matrix gets degraded and some gets deposited.
Cynthia St. Hilaire: So have you gone to the next steps of something like single cell seq and trying to see what kind of cells you're getting in this? Or even maybe inputting different, the scaffold is getting one differentiation protocol, but are you possibly able to prime IPS cells such that they're maybe halfway to a vascular cell, or halfway to a cardiomyocyte cell, and then put them in the bio-ink?
Brenda Ogle: That's a really interesting idea. I'm going to take that one.
Cynthia St. Hilaire: Give me an acknowledgment.
Brenda Ogle: So we've been thinking about that, the context of if expansion of IPS cells is the best way, for many cell types, how do we get multiple cell types and organize them? And you can imagine even just printing in specific areas, different cell types.
Cynthia St. Hilaire: Oh, sure.
Brenda Ogle: But the other thing we've thought about is delivering differentiation factor spatially.
So almost printing a cell, but then printing that. depot of a factor, in the area that we wanted or in an arrangement that we want, and then releasing it when we want. And it's challenging for stem cell differentiation, because you really need no release, and then basically zero order release for two or three days, and then no release again.
Cynthia St. Hilaire: Right.
Brenda Ogle: So it's a challenging drug delivery problem, but we've been thinking a lot about it. Now priming the cells beforehand is another interesting approach.
Cynthia St. Hilaire: Well, that's wonderful. I just want to congratulate you all again.
Brenda Ogle: Thank you so much for having us.
Cynthia St. Hilaire: Yeah, thank you so much.
Wei-Han Lin: Thank you so much.
Cynthia St. Hilaire: That's it for our highlights from the July issues of Circulation Research. Thank you so much for listening. Please check out the Circulation Research Facebook page and follow us on Twitter and on Instagram with the handle @CircRes and #discovercircres. Thank you to our guests, Dr. Brenda Ogle, Dr. Molly Kupfer and Wei-Han Lin. This podcast is produced by Rebecca McTavish and Ishara Ratnayake, edited by Melissa Stoner and supported by the editorial team of Circulation Research. Some of the copy texts for highlighted articles was provided by Ruth Williams. I'm your host Dr. Cindy St. Hilaire, and this is Discover CircRes, you're on the go source for the most up-to-date and exciting discoveries in basic cardiovascular research.