Genetic Regulation of Retinal Cells by Variation of Underlying Elastic Modulus

A major breakthrough in the development of engineering hydrogels for regenerative medicine occurred when it was noticed that when cells are plated on substrates of different stiffnesses undergo certain conformational changes in their cytoskeleton. Specifically, the tension in cytoskeleton of the cells on the substrates increased with stiffness. Many different cell types were noted to have a larger spread area and the presence of stress fibers. This was called, by Donald Ingber and Buckminster Fuller, tensegrity and is likened to a tent erected in different soils. The tent poles act as the skeleton with the outer covering as the membrane, the poles create tension on the pegs and, if they are not planted in firm ground, the tent snaps back. It has also been shown that pluripotent cells will develop into different specialized cells purely under the influence of substrate stiffness. This finding implies an alteration of the genetic regulation arising from the tension in the cytoskeleton. Hence, a structure that does not exist inside the nucleus is changing the transcription of DNA. How this non-chemical signaling occurs, as well as which genes are affected in which cells and what stiffness levels trigger these changes, is now what needs to be investigated. We began working with a retinal cell line to see if these unique cells change genetic expression in the same way, which might help to explain the buildup of scar tissue in the retina after reattachment surgery. This scar tissue (proliferative vitreoretiopathy), in many cases, leads to secondary retinal detachments. We discovered that there were many changes, both physically and genetically, that Müller cells undergo when exposed to varying levels of substrate stiffness. Changes found include varying expression of different extracellular matrix genes, an inverse relationship between the amount of phagocytosis and substrate stiffness, and determining that not only is protein expression of the cells different across substrates, the time of peak expression varies.