Engineered materials and the cellular microenvironment: a strengthening interface between cell biology and bioengineering

Abstract

Cells constantly probe and respond to a myriad of cues that are present in their local surroundings. The effects of soluble cues are relatively straightforward to manipulate, yet teasing apart how cells transduce signals from the extracellular matrix and neighboring cells has proven to be challenging due to the spatially and mechanically complex adhesive interactions. Over the years, advances in the engineering of biocompatible materials have enabled innovative ways to study adhesion-mediated cell functions, and numerous insights have elucidated the significance of the cellular microenvironment. Here, we highlight some of the major approaches and discuss the potential for future advancement.

Figure 1. Methods of ECM Patterning to Control Cell Shape and Adhesions

A. Microcontact printing process. A biomolecule is absorbed to the PDMS stamp surface. The stamp is then put in contact with the substrate. B & C. Immunofluorescent images of cells in flower (B) and star (C) shapes stained for F-actin (green), vinculin (red) and nuclei (blue) reproduced from [40]. D. B16 cell expressing β 3-integrin-GFP (green) labeled for actin (red) growing on vitronectin (blue) at the border between a uniform and a patterned substratum of 1 μm² dots. Note the redistribution of integrin receptors on the patterned substratum. Scale bars: 10 μm. Reproduced from [31]. E & F. Immunofluorescent micrographs of REF cells stained for vinculin (red), zyxin (blue) and actin (green). Cell adhering to 58 nm (E) and to 110 nm (F) nanopatterned surface for 24 h. Small inserts show each labeled protein at 2× magnification of the original images. Vinculin and zyxin can be seen to colocalize on the RGD nanodots spaced 58 nm apart but not 110 nm apart. Reproduced from [108]. G. Nanopatterning with block copolymer micelles allows the generation of substrates with a regularized pattern of gold nanoparticles. Block copolymer micelles with a polar core are used to hold a controllable amount of metal precursor in dilute solution. A substrate dipped into the solution comes out with a monolayer of micelles covering its surface. The inter-particle spacing can be controlled by using block copolymers with different lengths of blocks. The polymer is then removed by plasma treatment, leaving a quasihexagonal array of particles.

Figure 2. Elastic Substrates to Study Traction Forces

A. The displacement of fluorescent beads embedded within a polyacrylamide gel can be tracked beneath a migrating cell. The displacement can then be used to calculate the stress and strain fields due to the cell-generated traction forces. B. Similarly, cells can be grown on a bed of microposts (mPADs). Cell-generated traction forces deflect the posts, allowing the stress and strain fields to be calculated. C. Example of traction force microscopy. Mouse embryo fibroblasts (MEFs) marked with GFP-paxillin (green). Lower image shows a pseudo-colored map of traction magnitude calculated using Fourier-transform traction cytometry (FTTC). Units of color bar given in Pascals. White box indicates a region enlarged in a separate part of the figure (not shown). Reproduced from [65]. D. Scanning electron micrographs of human mesenchymal stem cells plated on PDMS micropost array with a post height of 6.10 μm. Lower image is a magnified version of boxed region. Scale bars are 100 μm in top image and 50 μm in lower image. Reproduced from reference [109].

Box Figure I. Chemical-mechano Interactions of Adhesion Maturation and Cytoskeletal Rearrangement

A. Schematic of adhesion maturation. (1) Nascent adhesion formation is initially driven by actin polymerization and consists of a complex of proteins linking integrins with actin. (2) Nascent adhesions elongate in response to actin-α-actinin-myosin II crosslinking. The signaling from adhesions activates small GTPases such as Rho that regulate further myosin II contractility and adhesion dynamics (3) Contractile forces generated by myosin II contribute to the further maturation of adhesions. B. During initial cell spreading, cell shape is constrained, Rac activity increases (Rho decreases), and adhesion formation is driven by actin polymerization (left). During later stages of cell spreading, the cell has spread out and flattened, forming mature adhesions and stress fibers and Rho activity is high (right).