Assessment of a Regenerative Therapy Strategy for Chondral Defects in Articular Cartilage
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Osteoarthritis is one of the leading causes of disability in adults over the age of 40 years, and there is currently no curative measure for the disease. As a result, there is a critical need for a strategy that can promote cartilage regeneration. Of particular interest is an injectable, in situ gelling hydrogel for cell encapsulation that can be delivered with low cytotoxicity to native chondrocytes and encapsulated cells. Adipose-derived stem cells are an attractive cell population for delivery because they secrete a wide array of soluble factors and they have the ability to differentiate down the chondrogenic lineage. Chemotaxis of passage 0 and passage 2 bovine chondrocytes toward bovine ASCs was measured using a modified Boyden chamber assay. Passage 0 chondrocyte migration was less than 3% of seeded cells for all conditions investigated with no significant differences between groups. Passage 2 chondrocytes exhibited significantly higher migration than P0 chondrocytes, but migration towards bASCs was actually lower than media controls. PDGF-BB and IGF-I promoted chemotaxis of passage 2 chondrocytes at 5 ng/mL and up to 100 ng/mL. Acrylate-poly(trimethylene carbonate)-block-poly(ethylene glycol)-block- poly(trimethylene carbonate)-acrylate (A-PTMC-PEG-PTMC-A) tri-block copolymers were synthesized from 3.4 and 20 kg/mol PEG-diol initiators. Methacrylate chondroitin sulphate (MCS) modified with collagen integrin peptides RGD, GLOGEN, and GVOGEA was blended with A-PTMC-PEG-PTMC-A to form hydrogels for ASC encapsulation. Conjugation of peptides onto MCS resulted in higher ASC viability in A-PTMC-PEG-PTMC-A/MCS hydrogels as assessed by LIVE/DEAD stain. The 20k/MCS hydrogels exhibited higher equilibrium water content, ultimate strain, and toughness, but lower equilibrium moduli than MCS and 3.4k/MCS hydrogels. In addition, 3.4k/MCS hydrogels were tougher than MCS hydrogels but had a comparable equilibrium modulus. The present work showed that ASCs do not encourage chemotaxis of bovine chondrocytes in 2-D culture. This finding provides early evidence that ASCs may be unsuitable for promoting chondrocyte migration in a cartilage defect model. The viability and mechanical measurements demonstrate that A-PTMC-PEG-PTMC-A/MCS hydrogels modified with integrin peptides can maintain ASC viability following encapsulation. However, further experiments are necessary to assess the suitability of peptide-modified A-PTMC-PEG-PTMC-A/MCS hydrogels as a viable cartilage regeneration strategy.