Tunable biomimetic hydrogel informs cell behavior

STEPHEN RIFFLE: Welcome to MRS Bulletin’s Materials News Podcast, providing breakthrough news & interviews with researchers on the hot topics in materials research. My name is Stephen Riffle.  

If you’ll bear with me for a moment, I’d like to start this one off with a thought experiment. Picture a busy city sidewalk with people hurriedly walking too and fro, feet colliding with the solid ground and propelling their owners onward. Now imagine this same busy street, but replace the pavement with mud. The hurried footfalls carry a different sound now, and the pace is much slower. It is a simple concept, but what’s demonstrated here is that our behavior heavily depends on our environment. How we move on concrete will necessarily differ from how we move on mud, or in snow, or in water. 

The same is true for the cells in our body. They too interact with the materials in their immediate environment—specifically a composite extracellular matrix material, or ECM for short.  The ECM contains a mixture of proteins and proteoglycans whose properties can significantly affect how a cell behaves. 

Researchers often don’t consider the ECM’s potential influence when studying cell behavior and how that behavior contributes to disease. Very often, researchers use two-dimensional cell culture models wherein cells are cultured atop plastic or glass dishes. 

SAM HERBERG: It makes a lot of sense to not subject cells to something that is so stiff, that is really not physiologic at all.

STEPHEN RIFFLE: That’s Samuel Herberg, an assistant professor of ophthalmology and visual sciences at SUNY upstate medical university in Syracuse, New York. Herberg is an engineer by training and has spent several years thinking about the material properties of the ECM. Herberg believes that culturing cells in an environment that reflects the material properties of the human body can help reveal new insights into cell biology and mechanisms of disease. To do that, his team has created a hydrogel using natural polymers. 

SAM HERBERG: So we basically set out to recreate a simplified version of the extracellular matrix environment using collagen and an elastin and hyaluronic acid. Obviously ECM proteins that are found in the body are very relevant in terms of disease modeling. And we made a choice to focus on those, but they suffer from one key drawback, which is the mechanical integrity. So most ECM type hydrogels are relatively soft to a point that are a little bit too soft to use. So there are some tweaks that we can use and we use a chemical crosslinking modality using UV lights and a photo initiator to subject these materials to a short burst of UV light to convert into chemically crosslinked side chains to give a little bit more mechanical integrity. And that is the basis of our three-dimensional hydrogel.

STEPHEN RIFFLE: Through UV and chemically induced crosslinking, Herberg’s team is able to finely tune their hydrogel’s stiffness. This allows them to then study diseases like primary open angle glaucoma, the world’s leading cause of vision loss. 

SAM HERBERG: So we've known about the disease for a very long time, but the precise reasons or the mechanisms that underlie the disease are not fully understood.

STEPHEN RIFFLE: What is known is that the aqueous humor, that fluid that fills up most of your eye, is constantly flowing into and out of the eye. This helps maintain appropriate intraocular pressure. But where the fluid exits the eye is a structure known as the trabecular meshwork, or TM. This structure contains a unique mixture of cells and extracellular matrix material. These components of the TM flex and contract to control the outflow of intraocular fluid. However, as glaucoma develops, the TM gets stiff and decreases fluid flow, leading to a rise in pressure and disease progression. 

SAM HERBERG: So a lot of our studies really focus on mechanical properties. So tissue stiffening is something that is widely acknowledged to play a role in a variety of diseases. But in certain diseases, there's a sort of a signature that tissue stiffness proceeds disease, and it can also drive its progression. And that seems to be happening in glaucoma or in outflow dysfunction. And with this stiffening, a variety of signaling pathways can be affected that can then lead to more contraction in more deposition of extracellular matrix material. That would be the feed forward cycle with the net result that at some point the whole machinery is basically collapsing.

STEPHEN RIFFLE: For primary open angle glaucoma, it’s not clear how the extracellular matrix and the signaling within trabecular meshwork cells change when the condition is developing and progressing. This may be because the extracellular matrix can greatly influence cell signaling, and thus to understand how a diseased cell behaves, you have to study it in the context of its extracellular matrix. This is where hydrogels can be extremely useful. Herberg and his team set out to study a specific signaling network known as YAP/TAZ. This well known signaling pathway has been implicated in glaucoma and is known to be sensitive to mechanical cues, like the stiffening of extracellular matrix. By culturing diseased and healthy trabecular meshwork cells inside or on top of hydrogels, the team could see how ECM stiffness affected YAP/TAZ signaling and disease phenotypes. 

SAM HERBERG: We identified some really interesting nuances that you would otherwise not see if you relied entirely on glass substrate. Through our collaborations with Duke University, we were able to get our hands on glaucoma cells. And one of our assays is to measure the ability of these cells to contract the gel. So the gel contraction in a way is a proxy for us to imagine what may happen in the tissue. If these cells are hyper contractile and they would kind of condense the ECM, that is what people discuss as a key feature of the dysfunction in the tissue. So if our model could recreate this, it would actually really validate the ECM three-dimensional approach. And so what we found was exactly that. So for the first time, all of a sudden we had something in our hands that recreated two key elements from the disease, the contraction, and the stiffening using this gel. And that had not been possible before. That's impossible to do this in 2D, and nobody had encapsulated these cells yet before in a hydrogel. And certainly not in a gel that is made of tissue components that are found there.

STEPHEN RIFFLE: By simulating the material properties of a cell’s natural environment, Herberg and his team have created a tool that may help researchers better understand cell signaling and diseases like primary open angle glaucoma. Such a tool could also be useful when trying to identify new therapeutic approaches that target key molecules involved in the regulation of ECM stiffness. 

This work was published in a recent issue of Frontiers in Cell and Developmental Biology. 

My name is Stephen Riffle from the Materials Research Society. For more news, log onto the MRS Bulletin website at mrsbulletin.org and follow us on twitter, @MRSBulletin. Don’t miss the next episode of MRS Bulletin Materials News – subscribe now. Thank you for listening.