I currently develop and use dynamically stiffening hydrogels particularly to study the stiffness mediated behavioral changes in skeletal muscle cells. Muscle is a highly dynamic tissue that can stiffen in different timescales following exercise, injury or during aging. We posit that the stiffness of the muscle significantly influences the behavior of resident satellite cells (MuSC), which are responsible for the regeneration and maintenance of the skeletal muscle. My work can be divided as follows: a) Developing novel hydrogels that can be stiffened in the presence of the cells and b) Understanding the cellular mechanisms that allow skeletal muscle cells to respond and adapt to changes in tissue stiffness.
a) Dynamically stiffening anthracene functionalized poly(ethylene glycol) hydrogels:
I exploited the [4+4] photocycloaddition of anthracene groups to generate robust in situ stiffening hydrogels (Figure a). Upon incorporation of the anthracene groups to PEG chains, hydrogels with controlled stiffness and a very high dynamic range (100 Pa to 100 kPa) can be achieved upon 365 nm light irradiation within 5-15 minutes. As the gel formation only depends on light irradiation, it is possible to stepwise stiffen these hydrogels in the presence of the cells. The photodimerization is biorthogonal, as it can be carried out in cell culture media. We are currently exploring these hydrogels to study stiffness-mediated changes in skeletal muscle cells upon injury as well as in cardiac fibroblasts following cardiac fibrosis.
b) Understanding mechanotransduction pathways of skeletal muscle cells using dynamically stiffening hydrogels:
Upon injury, skeletal muscle stiffness increases approximately 2-fold within 30 days and resident MuSCs show aberrant behavior at late post-injury stages. In order to understand if these behavioral changes are due to increased muscle stiffness, we currently investigate the mechanotransduction pathways in skeletal muscle cells using dynamically stiffening hydrogels. We are exploring the pathways that regulate nuclear translocation of key mechanosensitive transcription factors, such as Yes-associated protein (YAP) (Figure b).
Figure a) Anthracene photodimerization allows a simple but robust strategy to develop in situ stiffening hydrogels. The hydrogels have unprecedented dynamic range, as illustrated by the rheological trace of a 16 wt% PEG-anthracene hydrogel that can be stiffened from 10 to 50 kPa within 90 seconds to model changes in the heart stiffness upon cardiac fibrosis. Figure b) Muscle myoblast cells respond to changes in the stiffness of the hydrogel from 4 to 33 kPa by increasing their YAP nuclear localization.