Determinants of cell-material crosstalk at the interface: towards engineering of cell instructive materials

Abstract

The development of novel biomaterials able to control cell activities and direct their fate is warranted for engineering functional biological tissues, advanced cell culture systems, single-cell diagnosis as well as for cell sorting and differentiation. It is well established that crosstalk at the cell-material interface occurs and this has a profound influence on cell behaviour. However, the complete deciphering of the cell-material communication code is still far away. A variety of material surface properties have been reported to affect the strength and the nature of the cell-material interactions, including biological cues, topography and mechanical properties. Novel experimental evidence bears out the hypothesis that these three different signals participate in the same material-cytoskeleton crosstalk pathway via adhesion plaque formation dynamics. In this review, we present the relevant findings on material-induced cell response along with the description of cell behaviour when exposed to arrays of signals-biochemical, topographical and mechanical. Finally, with the aid of literature data, we attempt to draw unifying elements of the material-cytoskeleton-cell fate chain.

Figure 1.

Adhesion formation and cytoskeletal assemblies on patterns of biological signals. Decreasing the size of RGD-functionalized gold squares under 1 μm² results in poor recruitment of α5β1-integrin, altered actin assembly and a decreased rate of cell migration [40]. Decreasing adhesive spot density until a lateral spacing of 73 nm between integrin binding sites (RGD-functionalized gold nanoparticles 6 nm in diameter) inhibits focal contact formation [41]. Uniform distribution of RGD on functionalized substrates allows for cell adhesion when overcoming a threshold level depending on materials, bioconjugation and cell type. A minimum cluster of six RGD–gold nanoparticles (6 nm in diameter), each separated by 100 nm, was found to be the minimal number to activate cell adhesion, paxillin accumulation and subsequent focal adhesion formation [38]. (Online version in colour.)

Figure 2.

RGD motif on polymeric platforms. Spatial distribution of GRGDY on polycaprolactone (PCL) substrates [PCL-glutaraldehyde-(GA)-GRGDY] obtained by aminolysis in a 10% (w/w) 1,6-hexane-diamine/isopropanol solution and bioconjugation with 2% glutaraldehyde in 10 mM phosphate buffer (a). Confocal image for phalloidin staining of microfilaments (b) and scanning electron microscopy image (c) of NIH cells on PCL-GA-GRGDY substrates; a corrected surface density of about 1 mM cm⁻¹ was estimated as the number of molecules per area available sufficient for the integrin engagement with a characteristic rose-like clustering of the immobilized peptide at the interface. Profile of the variation of the RGD–polyethylene glycol (PEG)–acrylate concentration along the RGD gradient length (d); NIH3T3 cells adhered (e) and frames (f) of NIH3T3 migration on the RGD–PEG–acrylate scaffold (slope = 1 mM cm⁻¹; average RGD concentration = 1.5 mM). These results highlight the profound effect of spatial distribution of matricellular signals on the dynamics of focal formation and their directionality and orientation and ultimately on the shape and organization of the cytoskeleton. Therefore, the engineered gradient of adhesive molecules is another important ingredient of cell–material crosstalk because it can be instrumental to tuning cytoskeleton assembly and mechanics and therefore controlling cell functions and fate. (Online version in colour.)

Figure 3.

Illustration of cell–nanotopography interactions. Different combinations of feature sizes and depths might reduce the surface available for the cell membrane to establish adhesions. Looking at the pattern from the cell side, five integrins are necessary to connect the cytoskeleton to the extracellular environment [69], which requires integrin clusters of approximately 40 nm. Therefore, topographic features exceeding this dimension can be recognized by cells as ‘adhesive’. From the material side, ligand spacings below 60 nm proved to be sufficient to ensure cell-substrate binding [37]. In this case, inter-feature size exceeding this dimension might impair the formation of adhesive complexes. Moreover, surfaces are generally coated with serum proteins, which account for a macromolecular layer of 20–30 nm. Therefore, in order to make a surface not available for the establishment of cell adhesions, this has to be at least 40 nm away from the cell membrane, which comprises the distance of the protein layer and the extracellular domain of the integrins (H, height; FS, feature size; IF, inter-feature dimension). (Online version in colour.)

Figure 4.

Material–cytoskeleton crosstalk affects cell behaviour and fate with adhesion localization and composition, and cell-generated forces. Cell instructive materials aim at exploiting these characteristics in order to impart cell-specific orders by modulating adhesion and cytoskeletal assemblies through the delivery of biological, topographical and mechanical signals. (Online version in colour.)