Presentation of BMP‐2 from a Soft Biopolymeric Film Unveils its Activity on Cell Adhesion and Migration

Abstract

The dramatic effect of morphogenetic protein 2 (BMP-2), presented to cells in a “matrix bound” manner is described. BMP-2 is either delivered in solution or bound to a thin film made of poly(lysine) and hyaluronan. In this later case only, a striking effect of BMP-2 on cell adhesion and migration is observed. Cell biologists and biomaterials scientists are used to presenting cell adhesive ligands as they are in vivo: that is, in their insoluble form and in proximity to the cell adhesion site.1, 2 Intriguingly, this approach has barely been adopted for growth factors, usually presented to the cell in solution or as a releasable molecule, whereas growth factors in vivo are known to strongly interact and to bind with the components of the extracellular matrix.3, 4 In addition to these biochemical signals, the mechanical properties of the cell’s microenvironment are known to greatly impact cell behavior. To the best of our knowledge, the interplay between the mechanical properties of a biomaterial and the presentation of growth factor by the same material has not been investigated, to date. Over the years, growth factors have been shown to govern numerous cellular processes and are now recognized as promising therapeutic tools.5 Many tissue-engineering efforts to date have focused on modifying surfaces with extracellular matrix (ECM) proteins or adhesive peptides. Growth factors have been mixed with biomaterials to be released by diffusion.6, 7 In fundamental cell-biology research, there is also a large body of research dedicated to understanding the signalization pathways underlying cell stimulation by growth factors. Until now, in these cell biology studies, growth factors have mostly been added in solution to the culture medium of cells grown on stiff materials such as glass or tissue-culture polystyrene.8 The effects of such stiff materials on cellular processes can be quite subtle. Often, harsh serum starvation for about one day is necessary to render the cell quiescent, if any consequence of growth factor treatments on cellular behavior is to be observed.8, 9 Interestingly, an interplay between cell adhesion-receptor (e.g., integrin) signaling and growth-factor signaling has also been evidenced10 by plating cells on a given type of ECM protein to induce a specific signal while providing the growth factor in solution.11 The concept of presenting a growth factor “in the solid state”, which we will, here, refer to as “matrix-bound”, has been proposed in the pioneering work by Griffith et al.12 on tethered epidermal growth factor (EGF), but it has, surprisingly, been neglected for a long time. However, this mode of delivery is closer to physiological conditions, since most growth factors in ECM are bound to proteins and glycosaminoglycans.4 Thus, they are thus presented to cells in a matrix-bound manner. Only recently, this concept has been extended to other types of growth factor, including the vascular endothelial growth factor (VEGF)13 and the insulin-like growth factor 1.14 These recent experimental studies showed that the efficacy of the growth factors was improved, compared to the soluble form, and that their biological function was enhanced.13-15 The study of the family of bone morphogenetic proteins (BMPs) is an intensive field of research toward tissue-engineering applications16 and for fundamental insight into cell biology17 due to its physiological importance.18 BMPs play a crucial role in morphogenesis, tissue patterning, and regeneration after tissue damage, including cell differentiation.18, 19 In particular, BMP-2 is a highly potent morphogen that induces muscle precursors and mesenchymal stem-cell differentiation in bone cells.20, 21 Cell biologists have already noticed that BMP-2 added in solution to serum-starved cells plays also a role in early adhesive events, including adhesion and migration through actin reorganization.8, 22 The delivery of matrix-bound BMP-2 in a bioactive form is still a major challenge i