Thursday, 28 March 2013

Mesenchymal stem cell differentiation depends on ‘Grip’

A new study conducted by researchers at the University of Pennsylvania sheds new light on the differentiation process of mesenchymal stem cells (MSCs). Specifically, the researchers discovered that whether an MSC turns into a fat cell or a bone cell not only depends on the culture medium but on how well it can "grip" to it as well.

According to the researchers, many in the past have studied how stem cells behave when grown on a 2D material. However, the same cannot be said for 3D materials. This is due to most believing that the extra dimension won't actually influence the fate of the cultured stem cells.

In this study the researchers, led by Jason Burdick, focused on how 3D-scaffolds influence mechanotransduction, which is a term referring to the mechanisms by which cells convert mechanical stimulus into chemical activity. Simply put, they wanted to see if and how the surrounding environment affects the fate of mesenchymal stem cells.

“We’re trying to understand how material signals can dictate stem cell response,” said Burdick adding that 3D scaffolds aren't "inert structures", instead they significantly contribute to the differentiation process.

The researchers first obtained  human mesenchymal stem cells from the Lonza Corporation. Then, they cultured these cells in water-swollen polymer networks called hydrogels. There are many different types of hygrogels with the researchers choosing "covalently cross-linked gels, which contain irreversible chemical bonds."

Picture of a mesenchymal stem cell
A mesenchymal stem cell

Typically, MSCS cultured in 2D gels spread and pull differently, depending on the material's stiffness. Ultimately, different stiffness levels lead to different, specialised cells. MSCs grown in soft gels develop into fat-cells whereas MSCs grown in hard gels develop into bone cells.

However, the researchers were surprised to see that the same doesn't apply for 3D hydrogels, as experiments showed that in most cases, regardless of the gel's stiffness, the MSCs develop into fat cells.

“In most covalently cross-linked gels, the cells can’t spread into the matrix because they can’t degrade the bonds — they all become fat cells, That tells us that in 3D covalent gels the cells don’t translate the mechanical information the same way they do in a 2D system.” said Burdick.

Then, they decided to put this hypothesis to the test and used hygrogels with a different chemical structure. This time the hydrogels had a peptide that the stem cells could naturally degrade. Theoretically, the peptide would act as a "grip" that the stem cells could use, allowing them to become bone-cells.

To examine how well the stem cells were pulling on their environment, the researchers used a technique called 3D traction force microscopy. The technique was developed by Christopher Chen's who is also one of the study's authors. 3D traction force microscopy involves seeding the hydrogels with microscopic beads, then pinpointing their location before and after a cell is removed. Chen explains how the technique works:

“Because the gel is elastic and will relax back into its original position when you remove the cells, you can quantify how much the cells are pulling on the gel based on how much and which way it springs back after the cell is removed.”

The hypothesis proved to be correct, as the MSCs grown in the new hydrogels did manage to differentiate into bone cells. However, the researchers decided to do one more experiment to re-confirm their findings. This time they created a hydrogel similar to the previous one, which however was specially designed so that in the presence of light it would change chemically, disencouraging MSCs from anchoring on the scaffold, thus acting as the first type of hydrogels used in the study.

So, the researchers cultured the MSCs on the hydrogel and left it in the dark. One week later they saw that, as it was expected, the MSCs had begun transforming into bone-cells. Then they brought the hydrogel back to the light. As a result, the MSCs could no longer get a grip and started transforming to fat cells.

When we introduced these cross-links so they could no longer degrade the matrix, we saw an increase toward fat-like cells, even after letting them spread, This further supports the idea that continuous degradation is needed for the cells to sense the material properties of their environment and transduce that into differentiation signals.”  said Burdick.

The research team believes that their findings have implications in the field of tissue-engineering, as they help us understand how the properties of the surrounding environment impacts the fate of stem cells.

Khetan, S., Guvendiren, M., Legant, W., Cohen, D., Chen, C., & Burdick, J. (2013). Degradation-mediated cellular traction directs stem cell fate in covalently crosslinked three-dimensional hydrogels Nature Materials DOI: 10.1038/nmat3586

1 comment:

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