Microenvironment Design for Stem Cell Fate Determination - Tissue Engineering: Cell-Surface Interactions for Tissue Culture - page 238

Introducing Adhesion Peptides

One of the most studied sequences for biomaterial modifications is the arginine-
glycine-aspartic acid (RGD) sequence. It is a common adhesion motif in ECM
proteins, such as fibronectin, collagen and laminin [ 127 , 128 ], and various integrin
receptors bind directly to it [ 129 ]. Synthetic, RGD-containing peptides have been
incorporated into various materials and tissue engineering scaffolds to promote cell
adhesion and improve their cell function [ 130 ]. The interaction of these peptides
with cells has been shown to influence many processes within the cell, among
them proliferation, migration and differentiation [ 131 ]. Activation of these pro-
cesses is mediated by integrin-binding signal transduction to the cell interior,
which regulates organization of the cytoskeleton, activates kinase signaling

cascades, and modulates cell cycle and gene expression.
Hwang and colleagues showed that RGD-specific cell-matrix interactions could
promote the differentiation of hESCs into chondrocytes. Moreover, their results
suggested that RGD modification was even more efficient at inducing chondro-
genic differentiation than when exogenous type I collagen (which contains RGD)
was incorporated into the matrix [ 132 ]. Several studies indicated that RGD-
modified PLLA and hydroxyapatite scaffolds improved hMSCs adherence to the
matrix [ 133 , 134 ]. Additional studies indicated that RGD-decoration of PEG
hydrogels improved MSC viability [ 135 ] and promoted osteogenesis of MSCs in a
dosage-dependent manner, as judged by early and late bone marker expression.
It is suggested that rendering cell adhesion sites via introducing RGD into the
scaffold can help the maintenance of MSC osteogenic potential in the 3D PEG
hydrogels [ 103 ].
In another report, Li et al. [ 136 ] used a completely synthetic hydrogel system
presenting adhesive RGD motifs as a culture environment for self-renewal of
hESCs, replacing the conventional culture system on feeder layers of mouse
embryonic fibroblasts (MEFs). The researchers fabricated hydrogel composed of
poly(N-iso-propyl-acrylamide-co-acrylic acid) incorporating a semi-interpene-
trating network of cell adhesion RGD peptides. This culture system allowed the
simple and independent manipulation of cell adhesion ligand presentation and
matrix stiffness. ESCs that were cultured on the substrates adhered to the surface,
remained viable, maintained their morphology, and expressed markers of undif-
ferentiated hESCs [ 136 ].
In our group, RGD peptide immobilized onto alginate macro-porous scaffolds
was shown to enhance transforming growth factor-b1 (TGFb1)-induced hMSC
chondrogenic differentiation. The cell-matrix interactions facilitated by the
immobilized RGD peptide were shown to be an essential feature of the cell
microenvironment, allowing better cell accessibility to the chondrogenic-inducing
molecule TGFb1 (Fig. 4 )[ 81 ]. In another study conducted by our group, alginate
scaffolds were modified with an additional adhesive peptide, heparin binding
peptide (HBP), which is a target for cell syndecan interactions. The HBP was
immobilized onto alginate scaffolds, alone or in combination with RGD peptide.
The integration of these multiple cell-matrix interactions was shown to promote