The effect of material properties on cartilage-like tissue formation

Abstract

Articular cartilage is an avascular tissue type with very limited self-repair capacity, making it prone to degenerative diseases such as osteoarthritis (OA). The current therapeutic strategy for OA patients is predominantly directed towards pain relief rather than preventing degeneration and promoting the regeneration of cartilage tissue. Mesenchymal stromal cells (MSCs) have been proposed as a potential cell source for articular cartilage tissue engineering purposes. MSCs are mechanosensitive cells capable of sensing, transmitting, and responding to mechanical cues from their microenvironment through a process known as mechanotransduction. Utilizing the mechanotransductive behaviour of MSCs, cell differentiation can be directed towards a specific phenotype, in our case a chondrogenic cell. Considerable effort has been put into the identification of mechanotransductional regulators for chondrogenic differentiation of MSCs. However, due to the interdependency of the material properties by the crosslinking density, the effect of isolated material properties on cells remains unknown, and limits researchers to develop smart biomaterials for tissue engineering purposes through the concept of mechanobiology. This thesis will examine how material properties such as substrate stiffness and mesh size affect the chondrogenic differentiation potential of MSCs. A Unique approach based on the tunability of hydrogels to uncouple gel stiffness and mesh from each other will be used to assess the effect of isolated material properties. A hyaluronan based hydrogel with tyramine as a crosslinker (Ha-Tyr) enzymatically crosslinked with horseradish peroxidase (HRP) and hydrogen peroxide (H2O2) will be used within this study. As tunable parameters for the Ha- Tyr hydrogel, the polymer and H2O2 concentration were altered independently from each other. The evidence from this study confirmed that the gel stiffness of Ha-Tyr based hydrogels is polymer concentration and H2O2 concentration dependent. The tunability of these two parameters independently from each other enabled the production of different gel conditions having matching bulk stiffness. Unfortunately, the mesh size determination was not precise enough for complete uncoupling of the gel stiffness and mesh size from each other. Despite its limitations, the findings indicate that the mesh size shows a trend towards larger mesh size with increasing polymer concentration and lower H2O2 concentration. Thereby, indicating that difference in crosslinking density may have been the driving force for the observed trend. Translation of the effect of material properties on the chondrogenic potential of MSCs indicated towards reduced cartilage-like matrix deposition with increasing crosslinking density. These findings suggest that in general the matrix deposition of cartilage-like tissue is driven by the crosslinking density rather than gel stiffness. Furthermore, gene expression levels for ECM remodelling genes like MMP1 and MMP3 showed increased expression patterns with higher crosslinking density, which hints towards the fact that these MMPs may have played a pivotal role in the observed matrix deposition. The information obtained helps to identify mechanotransductional regulators that could be used for the development of smart biomaterials for tissue engineering purposes. Ultimately, it brings us a step closer to the development of functional cartilage tissue that could be used as a possible therapeutic strategy for OA patients.