Andrew Baldwin

We previously developed a 14-day culture protocol under potentially GMP, chemically defined conditions, to generate chondro-progenitors from human embryonic stem cells (hESCs). In vivo work has confirmed the cartilage repair capacity of these cells in a nude rat osteochondral defect model. Aiming to enhance hESC-chondrogenesis we screened a range of extracellular matrix (ECM) molecules for their ability to support differentiation of hESCs towards chondrocytes. We identified two novel ECM protein fragments that supported hESC-chondrogenesis: Fibronectin III (fibronectin 7-14 protein fragments including the RGD domain, syndecan binding domain and heparin binding domain); fibrillin-1 (FBN1) fragment PF8 (encoded by exons 30–38, residues 1238–1605, which contains the RGD motif but not heparin binding site). These two protein fragments support hESC-chondrogenesis compared with the substrates routinely used previously (a mixture of fibronectin and gelatin) in our directed chondrogenic protocol. We have identified recombinant fibronectin fragment (FN III) and FBNI fragment (PF8) as alternative coating substrates to promote expression of genes known to regulate chondrocytes and code for chondrocyte extracellular matrix components. These recombinant protein fragments are likely to have better batch to batch stability than full length molecules especially where extracted from tissue/serum.

Here, we show that epithelial–mesenchymal status influences how cells deposit extracellular matrix. Retinal pigmented epithelial (RPE) cells that expressed high levels of E-cadherin and had cell–cell junctions rich in zona occludens (ZO)-1, b-catenin and heparan sulfate, required syndecan-4 but not fibronectin or protein kinase C a (PKCa) to assemble extracellular matrix (fibrillin microfibrils and perlecan). In contrast, RPE cells that strongly expressed mesenchymal smooth muscle a-actin but little ZO-1 or E-cadherin, required fibronectin (like fibroblasts) and PKCa, but not syndecan-4. Integrins a5b1 and/or a8b1 and actomyosin tension were common requirements for microfibril deposition, as was heparan sulfate biosynthesis. TGFb, which stimulates epithelial–mesenchymal transition, altered gene expression and overcame the dependency on syndecan-4 for microfibril deposition in epithelial RPE cells, whereas blocking cadherin interactions disrupted microfibril deposition. Renal podocytes had a transitional phenotype with pericellular b-catenin but little ZO-1; they required syndecan-4 and fibronectin for efficient microfibril deposition. Thus, epithelial–mesenchymal status modulates microfibril deposition.