Mechanical loading regimes affect the anabolic and catabolic activities by chondrocytes encapsulated in PEG hydrogels

Mechanical loading of cell-laden synthetic hydrogels is one strategy for regenerating functional cartilage. This work tests the hypothesis that type of loading (continuous vs intermittent) and timing when loading is applied (immediate vs delayed) influence anabolic and catabolic activities of chondrocytes when encapsulated in poly(ethylene glycol) (PEG) hydrogels.

Primary bovine chondrocytes encapsulated in PEG hydrogels were subjected to unconfined dynamic compressive strains applied continuously or intermittently for 1 week (i.e., immediate) or intermittently for 1 week but after a 1 week free-swelling (FS) period (i.e., delayed). Anabolic activities were assessed by gene expression for collagen II and aggrecan (AGC) and extracellular matrix (ECM) deposition by (immuno)histochemistry. Catabolic activities were assessed by gene expression for matrix metalloproteinases, MMP-1, 3, and 13.

Intermittent loading (IL) upregulated ECM and MMP expressions, e.g., 2-fold, 16-fold and 8-fold for collagen II, MMP-1, MMP-3, respectively. Continuous loading upregulated AGC expression 1.5-fold but down-regulated MMP-1 (3-fold) and -3 (2-fold) expressions. For delayed loading, chondrocytes responded to FS conditions by down-regulating MMP expressions (P<0.01), but were less sensitive to loading when applied during week 2. Spatially, deposition of ECM molecules was dependent on the timing of loading, where immediate loading favored enhanced collagen II deposition.

The type and timing of dynamic loading dramatically influenced ECM and MMP gene expression and to a lesser degree matrix deposition. Our findings suggest that early applications of IL is necessary to stimulate both anabolic and catabolic activities, which may be important in regenerating and restructuring the engineered tissue long-term.