Photopolymerized poly(ethylene glycol) (PEG) hydrogels were employed as a base system to study cell response to loading in the absence of cell-material interactions. To study mechanoreceptors, matrix analogs (e.g., RGD, a common cell adhesion ligand) were systematically incorporated into the PEG gels. Chondrocytes were encapsulated in PEG and PEG+RGD hydrogels and subjected to dynamic compressive strains. Cell response was measured by anabolic and catabolic gene expression as well as matrix production.
In the absence of loading, anabolic gene expression by chondrocytes encapsulated in PEG gels was stimulated (e.g., aggrecan expression increased 6-fold) while catabolic gene expression was significantly inhibited (e.g., matrix metalloproteinase-3 decreased by 2 orders of magnitude) by day 10 of culture. When PEG constructs were subjected to dynamic loading (0.3 Hz, 1 hr on/1hr off for 12 hrs, 12 hrs off), both anabolic and catabolic gene expression were enhanced (e.g., aggrecan and MMP-3 increased by 2.2 and 7-fold, respectively, by day 7 of culture) suggesting that loading may induce remodeling of the newly deposited tissue. The addition of RGD into PEG gels up to 0.4 mM RGD did not affect chondrocyte phenotype as measured by the ratio of collagen II/I in the absence of loading. The addition of 0.4 mM RGD led to a significant reduction in proteoglycan synthesis compared to PEG gels in the absence of loading. However, when PEG+RGD gels were subjected to dynamic loading (0.3 Hz, continuous for 2 days), proteoglycan synthesis was stimulated 2-fold with 0.4 mM RGD compared to unloaded PEG+RGD gels suggesting that RGD is acting as a mechanoreceptor to stimulate matrix production.
Together our results demonstrate that loading and the incorporation of matrix analogs have a profound effect on chondrocyte response. We are currently exploring a range of loading regimes and other matrix analogs (e.g., glycosaminoglycans) toward elucidating biomechanical cues that stimulate tissue growth.