Development of chimeric forms of the matrix metalloproteinase 2 collagen binding domain as artificial membrane binding proteins for targeting stem cells to cartilage lesions in osteoarthritic joints.

Targeting stem cells to cartilage lesions has the potential to enhance engraftment and chondrogenesis. Denatured type II collagen fibrils (gelatin) are exposed in lesions at the surface of osteoarthritic articular cartilage and are therefore ideal target sites. We have designed and investigated chimeric mutants of the three modules of the MMP-2 collagen binding domain (CBD) as potential ligands for stem cell targeting. We expressed full-length CBD for the first time and used it to identify the most important amino acid residues for binding to gelatin. Module 2 of CBD had the highest affinity binding to both Type I and Type II gelatin, whereas module 1 showed specificity for type II gelatin and module 3 for type I gelatin. We went on to generate chimeric forms of CBD consisting of three repeats of module 1 (111), module 2 (222) or module 3 (333). 111 lacked solubility and could not be further characterised. However 222 was found to bind to type II gelatin 14 times better than CBD, suggesting it would be optimal for attachment to cartilage lesions, whilst 333 was found to bind to type I gelatin 12 times better than CBD, suggesting it would be optimal for attachment to lesions in type I collagen-rich tissues. We coated 222 onto the external membrane of Mesenchymal Stem Cells and demonstrated higher attachment of the coated cells to type II gelatin than uncoated cells. We conclude that the three modules of CBD each have specific biological properties that can be exploited for targeting stem cells to cartilage lesions and other pathological sites.