# The Role of Extracellular Matrix (ECM) Adhesion Motifs in Functionalised Hydrogels
## 5 ECM-Derived Peptide Sequences and Their Use in Biomimetic Hydrogel Functionalisation
### Collagen: GFOGER Motif  
The \(\alpha\)1\(\beta\)1 and \(\alpha\)2\(\beta\)1 integrins are known to interact with a variety of collagen subtypes. The type I collagen \(\alpha\)2\(\beta\)1 integrin recognition site is reported to contain the amino acid sequence (GFOGERGVEGPOGPA), located at residues 502-516 of the \(\alpha\)1(I) chain [99]. Further work confirmed that the N-terminal hexapeptide sequence, GFOGER, within the 15-residue amino acid sequence, is entirely responsible for providing integrin recognition (residues 506-511). It has been shown that the GFOGER sequence can promote cell adhesion through both the \(\alpha\)1\(\beta\)1 and \(\alpha\)2\(\beta\)1 integrin receptors [99]. The GFOGER sequence has been used extensively throughout the literature as a biomimetic motif to modify hydrogels, with the aim of controlling vital cellular processes conducted by the ECM. In a 2014 study, hMSCs were encapsulated in degradable and non-degradable PEG hydrogels, modified with the sequence GPC(GPP)\({}_{5}\)-GFOGER-(GPP)\({}_{5}\)GPC-NH2 [100]. Cell proliferation following hydrogel encapsulation was monitored using a bromodeoxyuridine assay, where proliferation was found to be four- fold higher in GFOGER-modified degradable gels compared with degradable gels without any adhesion motif (\(p\) = 0.017) [100]. Proliferation was also two-fold higher in degradable GFOGER hydrogels compared with RGD-modified degradable hydrogels (\(p\) = 0.069) [100]. The deposition of glycosaminoglycans within the hydrogels was measured both quantitatively and qualitatively as a marker for chondrogenic differentiation. The GFOGER-modified degradable hydrogels promoted the highest glycosaminoglycan content, as well as higher expression of the chondrogenic markers type II collagen and aggrecan. These results suggest that GFOGER-modified hydrogels can successfully support cell proliferation and the deposition of a chondrogenic ECM, and could be employed for MSC-based treatments of cartilage lesions [100]. A collagen mimetic peptide containing the GFOGER sequence ((GPO)\({}_{4}\)GFOGER(GPO)\({}_{4}\) GCG) has also been incorporated into PEG hydrogels and investigated for the ability to enhance chondrogenic differentiation of MSCs [101]. PEG hydrogels with the integrin specific GFOGER collagen sequence were shown to display improved cell viability and chondrogenic differentiation [101]. This study shows similar application, outcome measures and results compared with [PERSON] et al. [100], the only difference being the shorter peptide length of (GPO)\({}_{4}\)GFOGER(GPO)\({}_{4}\)GCG, suggesting that it could deliver similar results. Synthetic PEG hydrogels containing the GFOGER motif have also been investigated for their ability to stimulate survival and osteogenic differentiation of human-bone-marrow-derived stem cells (hBMSCs) [102]. It was shown that GFOGER-modified hydrogels stimulated elevation of FAK phosphorylation compared with inactive GAOGER-modified hydrogel controls, suggesting that this peptide may harbour the potential to biologically mimic essential ECM cell signalling mechanisms [102]. GFOGER-modified hydrogels were shown to support cell adhesion, signalling and osteogenic reparative functions when transplantedinto murine bone defects, replicating some of the properties of the ECM in vitro and in vivo [102]. [PERSON] et al. [103] prepared an in situ forming polymer hydrogel, conjugated with GFOGER, named polyethylene glycol-b-polycaprolactone (GFOGER-PEG-PCL), utilising various GFOGER concentrations. The expression of \(\beta\)1, \(\alpha\)2 and \(\alpha\)11 integrins in encapsulated BMSCs was monitored as a measure of adhesive capacity. In the polymer hydrogel modified with 0.8 mM GFOGER, a strong expression of all three integrins was detected. Following 7 days culture, cell proliferation in the PEG-PCL hydrogel modified with 0.8 mM GFOGER showed a four-fold increase in DNA production, compared to controls. The ability of the hydrogel to regenerate articular cartilage and subchondral bone was also assessed.